diff --git a/docs/analysis/PHASE3_BASELINE_AND_CANDIDATES.md b/docs/analysis/PHASE3_BASELINE_AND_CANDIDATES.md
index e7ae33b3..925beaf0 100644
--- a/docs/analysis/PHASE3_BASELINE_AND_CANDIDATES.md
+++ b/docs/analysis/PHASE3_BASELINE_AND_CANDIDATES.md
@@ -303,18 +303,20 @@ static inline void* tiny_alloc_gate_fast(size_t size)
 ---
 
 ### Phase 3 D3: Alloc Gate Specialization (MEDIUM PRIORITY)
-**Target**: `tiny_alloc_gate_fast()` for LEGACY-only route
+**Target**: `tiny_alloc_gate_fast()` の分岐形を最短化（MIXED 向け）
 **Expected Gain**: +1-2%
 **Risk**: LOW
 **Effort**: 2-3 hours
 
 **Implementation**:
-1. Create `tiny_alloc_gate_fast_legacy()` specialized variant
-2. Eliminate ROUTE_POOL_ONLY and ROUTE_TINY_FIRST branches
-3. Use in MIXED profile where all classes are LEGACY
+1. New ENV gate: `HAKMEM_ALLOC_GATE_SHAPE=0/1`
+2. `tiny_route_get()` を避け、`g_tiny_route[]` の直接参照に置換（release logging branch を回避）
+3. `ROUTE_POOL_ONLY` は必ず尊重（`HAKMEM_TINY_PROFILE=hot/off` を壊さない）
 4. A/B test: BASELINE vs D3
 
-**ENV Gate**: `HAKMEM_ALLOC_GATE_LEGACY_ONLY=1` (default: 0)
+**Design**: `docs/analysis/PHASE4_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md`
+
+**ENV Gate**: `HAKMEM_ALLOC_GATE_SHAPE=0/1` (default: 0)
 
 ---
 
@@ -376,7 +378,7 @@ static inline void* tiny_alloc_gate_fast(size_t size)
 
 2. **Optional**: Phase 3 D3 (Alloc gate specialization) - pending perf validation
    - Only proceed if perf shows ≥5% self% in alloc gate
-   - ENV: `HAKMEM_ALLOC_GATE_LEGACY_ONLY=0/1`
+   - ENV: `HAKMEM_ALLOC_GATE_SHAPE=0/1`
 
 3. **Phase 4 Planning**: If no more 5%+ targets, prepare Phase 4 roadmap
 
diff --git a/docs/analysis/PHASE3_CACHE_LOCALITY_NEXT_INSTRUCTIONS.md b/docs/analysis/PHASE3_CACHE_LOCALITY_NEXT_INSTRUCTIONS.md
index be33154d..96c7e750 100644
--- a/docs/analysis/PHASE3_CACHE_LOCALITY_NEXT_INSTRUCTIONS.md
+++ b/docs/analysis/PHASE3_CACHE_LOCALITY_NEXT_INSTRUCTIONS.md
@@ -85,9 +85,9 @@ perf report --stdio
 
 狙い: Mixed 本線の固定構成に合わせ、alloc gate の分岐を削って 1–2% 詰める。
 
-- 設計メモ: `docs/analysis/PHASE3_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md`
 - 実装指示書: `docs/analysis/PHASE4_ALLOC_GATE_SPECIALIZATION_NEXT_INSTRUCTIONS.md`
-- ENV: `HAKMEM_ALLOC_GATE_LEGACY_ONLY=0/1`（default 0）
+- 設計メモ（最新版）: `docs/analysis/PHASE4_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md`
+- ENV: `HAKMEM_ALLOC_GATE_SHAPE=0/1`（default 0）
 - 注意: “safe enable 判定” を必ず入れて、ENV 組み合わせで壊れないようにする
 
 ## 次候補（perf で 5% 超なら着手）
diff --git a/docs/analysis/PHASE3_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md b/docs/analysis/PHASE3_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md
index 8adbe880..580b9e05 100644
--- a/docs/analysis/PHASE3_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md
+++ b/docs/analysis/PHASE3_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md
@@ -1,5 +1,8 @@
 # Phase 3 D3: Alloc Gate Specialization（Mixed “LEGACY-only” 最短化）設計メモ
 
+> NOTE (2025-12-13): この設計メモは **後続の Phase 4 D3** により置き換えられました。  
+> 最新の設計・ENV・実装指示は `docs/analysis/PHASE4_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md` を参照してください。
+
 ## 目的
 
 Mixed 本線（`MIXED_TINYV3_C7_SAFE`）では、
@@ -84,4 +87,3 @@ Mixed 10-run（推奨: 20M iters）:
 
 - `HAKMEM_ALLOC_GATE_LEGACY_ONLY=0`（即 OFF）
 - 箱は独立させ、既存 gate を汚さない（境界 1 箇所差し替え）
-
diff --git a/docs/analysis/PHASE3_FINALIZATION_SUMMARY.md b/docs/analysis/PHASE3_FINALIZATION_SUMMARY.md
index 6d44152d..9d2a9ba9 100644
--- a/docs/analysis/PHASE3_FINALIZATION_SUMMARY.md
+++ b/docs/analysis/PHASE3_FINALIZATION_SUMMARY.md
@@ -238,14 +238,14 @@ IF mean_gain >= +1.0% AND median_gain >= +0.0%:
 
 **Requirement**: perf validation showing `tiny_alloc_gate_fast` self% ≥ 5%
 
-**Design**: `docs/analysis/PHASE3_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md`
+**Design**: `docs/analysis/PHASE4_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md`
 
 **Strategy**: Specialize alloc gate for fixed MIXED configuration
 - Eliminate dynamic checks
 - Inline hot paths
 - Reduce branch complexity
 
-**ENV**: `HAKMEM_ALLOC_GATE_LEGACY_ONLY=0/1`
+**ENV**: `HAKMEM_ALLOC_GATE_SHAPE=0/1`
 
 **Decision Criteria**:
 - IF perf shows ≥5% self% in alloc gate → Proceed with D3
diff --git a/docs/analysis/PHASE4_ALLOC_GATE_SPECIALIZATION_NEXT_INSTRUCTIONS.md b/docs/analysis/PHASE4_ALLOC_GATE_SPECIALIZATION_NEXT_INSTRUCTIONS.md
index 75bd423d..1fee4f9f 100644
--- a/docs/analysis/PHASE4_ALLOC_GATE_SPECIALIZATION_NEXT_INSTRUCTIONS.md
+++ b/docs/analysis/PHASE4_ALLOC_GATE_SPECIALIZATION_NEXT_INSTRUCTIONS.md
@@ -29,24 +29,16 @@ perf report --stdio --no-children
 
 ## Step 1: D3 実装（Box Theory）
 
-設計メモ（SSOT）: `docs/analysis/PHASE3_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md`
+設計メモ（SSOT）: `docs/analysis/PHASE4_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md`
 
 ### 箱割り（必須）
 
 - L0: ENV（戻せる）
-  - `HAKMEM_ALLOC_GATE_LEGACY_ONLY=0/1`（default 0）
-- L1: Safe-Enable 判定（境界 1 箇所）
-  - Mixed 本線が “LEGACY-only” であることを 1 回だけ確認し、成立した場合のみ fast 経路へ
-  - Learner ON の場合は必ず disable（route が動的更新されるため）
-- L2: Integration（境界 1 箇所）
-  - `tiny_alloc_gate_fast()` の入口でのみ差し替え（既存の意味は変えない）
-
-### Safe-Enable 条件（Fail-Fast ではなく “自動 disable”）
-
-以下のどれかに該当したら D3 は **即 disable** して既存経路へフォールバック:
-- learner が有効（例: `HAKMEM_SMALL_LEARNER_V7_ENABLED=1`）
-- route_kind に LEGACY 以外が混入（V7/MID/ULTRA/TINY_FIRST 等）
-- そのほか Mixed 本線の固定前提が崩れている
+  - `HAKMEM_ALLOC_GATE_SHAPE=0/1`（default 0）
+- L1: Gate Shape（境界 1 箇所）
+  - `tiny_alloc_gate_fast()` 内の分岐形だけを変更（意味は変えない）
+  - `tiny_route_get()` を呼ばずに `g_tiny_route[]` を直接読む（release logging branch を避ける）
+  - **Safety**: `g_tiny_route[ci] == ROUTE_POOL_ONLY` のときは必ず `NULL` を返す（`HAKMEM_TINY_PROFILE=hot/off` を壊さない）
 
 ## Step 2: A/B（GO/NO-GO）
 
@@ -54,12 +46,12 @@ Mixed 10-run（推奨: iter=20M, ws=400, 1T）:
 
 - Baseline:
 ```bash
-HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE HAKMEM_ALLOC_GATE_LEGACY_ONLY=0 \
+HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE HAKMEM_ALLOC_GATE_SHAPE=0 \
   ./bench_random_mixed_hakmem 20000000 400 1
 ```
 - Optimized:
 ```bash
-HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE HAKMEM_ALLOC_GATE_LEGACY_ONLY=1 \
+HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE HAKMEM_ALLOC_GATE_SHAPE=1 \
   ./bench_random_mixed_hakmem 20000000 400 1
 ```
 
diff --git a/docs/analysis/PHASE4_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md b/docs/analysis/PHASE4_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md
new file mode 100644
index 00000000..b8ab0e4a
--- /dev/null
+++ b/docs/analysis/PHASE4_D3_ALLOC_GATE_SPECIALIZATION_1_DESIGN.md
@@ -0,0 +1,417 @@
+# Phase 4 D3: Alloc Gate Specialization 設計メモ
+
+## 目的
+
+`tiny_alloc_gate_fast()` のルーティング分岐を MIXED 向けに特化（LEGACY 優先パス）
+
+**背景**:
+- Phase 3 完了: +8.93% cumulative gain (37.5M → 51M ops/s)
+- Perf analysis: `tiny_alloc_gate_fast` at **12.75% self** (HIGH priority)
+- MIXED workload: 99% が LEGACY route（MID_V3 OFF）
+- 現状: 全 route (LEGACY/ULTRA/MID/V7) をスイッチで分岐 → 予測失敗コスト高
+
+## 観察
+
+### Current State (Phase 3 Baseline)
+- `tiny_alloc_gate_fast`: 12.75% self + children overhead
+- MIXED ワークロード特性:
+  - 99% が LEGACY route（`HAKMEM_MID_V3_ENABLED=0`）
+  - C0-C7 全体で uniform branching → prediction miss
+- 既存最適化（Phase 2 B3）:
+  - `malloc_tiny_fast_for_class()` 内で LEGACY-first branching 実装済み
+  - しかし `tiny_alloc_gate_fast()` の routing policy 分岐は最適化されていない
+
+### Bottleneck Analysis
+
+**Current flow** (`core/box/tiny_alloc_gate_box.h:139-217`):
+```c
+void* tiny_alloc_gate_fast(size_t size) {
+    int class_idx = hak_tiny_size_to_class(size);
+    if (class_idx < 0 || class_idx >= TINY_NUM_CLASSES) return NULL;
+
+    TinyRoutePolicy route = tiny_route_get(class_idx);  // ← Policy lookup
+
+    // Branching on route policy (uniform dispatch, poor prediction)
+    if (route == ROUTE_POOL_ONLY) return NULL;
+
+    void* user_ptr = malloc_tiny_fast_for_class(size, class_idx);
+
+    if (route == ROUTE_TINY_ONLY) {
+        // Hot path (99% of Mixed traffic)
+        return user_ptr;
+    }
+
+    // ROUTE_TINY_FIRST: fallback allowed
+    return user_ptr;
+}
+```
+
+**Problem**:
+- Policy lookup overhead: `tiny_route_get()` call every allocation
+- Branch on `route == ROUTE_POOL_ONLY`: rare but evaluated every time
+- Branch on `route == ROUTE_TINY_ONLY` vs `ROUTE_TINY_FIRST`: Mixed default は TINY_FIRST（挙動差はほぼ無い）
+- Total cost: ~2-3 branches + 1 policy lookup per allocation
+
+**Expected savings**:
+- Eliminate policy lookup for known-LEGACY workloads
+- Convert policy branches to LIKELY-hinted checks
+- Reduce instruction count by 5-10 per allocation
+
+## 実装アプローチ
+
+### Strategy: LEGACY-first with Static Route Assumption
+
+**Pattern**: Similar to Phase 2 B3 (routing shape optimization)
+- Reference: `core/front/malloc_tiny_fast.h:262-278`
+- Proven approach: LIKELY hint + cold helper for rare routes
+- Expected branch prediction improvement: 75% miss rate → <5% miss rate
+
+### L0: Env（戻せる）
+
+- `HAKMEM_ALLOC_GATE_SHAPE=0/1` (default: 0, OFF)
+- Opt-in で特化パスを有効化（常時有効化は慎重）
+- Rollback: ENV=0 で即座に既存経路へ復帰
+
+### L1: SpecializedGateBox（境界: 1箇所）
+
+#### Optimized Gate Structure
+
+**Before** (current):
+```c
+void* tiny_alloc_gate_fast(size_t size) {
+    int class_idx = hak_tiny_size_to_class(size);
+    if (class_idx < 0 || class_idx >= TINY_NUM_CLASSES) return NULL;
+
+    TinyRoutePolicy route = tiny_route_get(class_idx);
+
+    if (route == ROUTE_POOL_ONLY) return NULL;
+
+    void* user_ptr = malloc_tiny_fast_for_class(size, class_idx);
+
+    if (route == ROUTE_TINY_ONLY) return user_ptr;
+
+    return user_ptr;  // ROUTE_TINY_FIRST
+}
+```
+
+**After** (optimized for MIXED):
+```c
+void* tiny_alloc_gate_fast(size_t size) {
+    int class_idx = hak_tiny_size_to_class(size);
+    if (__builtin_expect(class_idx < 0 || class_idx >= TINY_NUM_CLASSES, 0)) {
+        return NULL;
+    }
+
+    // Phase 4 D3: LEGACY-first gate specialization (ENV gated)
+    if (TINY_HOT_LIKELY(alloc_gate_shape_enabled())) {
+        // MIXED fast path: Avoid tiny_route_get() overhead.
+        // NOTE: We do NOT assume TINY_ONLY vs TINY_FIRST; both return user_ptr.
+        // Safety: still honor POOL_ONLY if configured via HAKMEM_TINY_PROFILE (e.g., "hot"/"off").
+        if (__builtin_expect(g_tiny_route[class_idx & 7] == ROUTE_POOL_ONLY, 0)) {
+            return NULL;
+        }
+        // Direct to malloc_tiny_fast_for_class, skip tiny_route_get()
+        return malloc_tiny_fast_for_class(size, class_idx);
+    }
+
+    // Original path (backward compatible)
+    TinyRoutePolicy route = tiny_route_get(class_idx);
+
+    if (__builtin_expect(route == ROUTE_POOL_ONLY, 0)) return NULL;
+
+    void* user_ptr = malloc_tiny_fast_for_class(size, class_idx);
+
+    if (TINY_HOT_LIKELY(route == ROUTE_TINY_ONLY)) return user_ptr;
+
+    return user_ptr;
+}
+```
+
+#### Branch Prediction Impact
+
+**Current** (uniform branching):
+- Policy lookup: Always executed (1 function call overhead)
+- `route == ROUTE_POOL_ONLY`: 0% hit rate (but checked every time)
+- `route == ROUTE_TINY_ONLY`: 99% hit rate (but no LIKELY hint)
+- Total overhead: ~10-15 cycles per allocation
+
+**Optimized** (LEGACY-first):
+- ENV check: 99% cached (< 1 cycle amortized)
+- Direct path: Skip `tiny_route_get()` (and its release logging branch) (save ~5-7 cycles)
+- LIKELY hint: CPU predictor trained to expect fast path
+- Total savings: ~8-12 cycles per allocation
+
+**Expected gain**: +1-2% on MIXED (conservative estimate based on 12.75% self%)
+
+### 実装指示
+
+#### File 1: `core/box/tiny_alloc_gate_shape_env_box.h` (新規)
+
+**Role**: ENV gate for alloc gate shape optimization
+
+**API**:
+```c
+// ENV gate: HAKMEM_ALLOC_GATE_SHAPE=0/1 (default: 0)
+static inline int alloc_gate_shape_enabled(void) {
+    static int g_enable = -1;  // Lazy init sentinel
+    if (__builtin_expect(g_enable == -1, 0)) {
+        const char* e = getenv("HAKMEM_ALLOC_GATE_SHAPE");
+        g_enable = (e && *e && *e != '0') ? 1 : 0;
+    }
+    return g_enable;
+}
+```
+
+**Integration**: Header-only, single-responsibility (ENV caching only)
+
+#### File 2: Modify `core/box/tiny_alloc_gate_box.h` (既存)
+
+**Location**: `tiny_alloc_gate_fast()` function (lines 139-217)
+
+**Changes**:
+1. Include new ENV box header:
+   ```c
+   #include "tiny_alloc_gate_shape_env_box.h"
+   ```
+
+2. Add LEGACY-first fast path before existing route dispatch:
+   ```c
+   // Phase 4 D3: LEGACY-first gate specialization
+   if (TINY_HOT_LIKELY(alloc_gate_shape_enabled())) {
+       // Skip policy lookup for MIXED (ROUTE_TINY_ONLY assumption)
+       return malloc_tiny_fast_for_class(size, class_idx);
+   }
+   ```
+
+3. Add LIKELY hints to existing branches (backward compatible path):
+   ```c
+   if (__builtin_expect(route == ROUTE_POOL_ONLY, 0)) return NULL;
+
+   void* user_ptr = malloc_tiny_fast_for_class(size, class_idx);
+
+   if (TINY_HOT_LIKELY(route == ROUTE_TINY_ONLY)) return user_ptr;
+   ```
+
+**Safety**:
+- ENV gate ensures opt-in behavior
+- Fallback path unchanged (existing validation/diagnostics preserved)
+- No algorithmic changes (only branch shape optimization)
+
+## A/B テスト
+
+### Test Configuration
+
+**Workload**: Mixed (10-run, 20M iters, ws=400)
+- Baseline: `HAKMEM_ALLOC_GATE_SHAPE=0`
+- Optimized: `HAKMEM_ALLOC_GATE_SHAPE=1`
+
+**Commands**:
+```bash
+# Baseline
+HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE HAKMEM_ALLOC_GATE_SHAPE=0 \
+  ./bench_random_mixed_hakmem 20000000 400 1
+
+# Optimized
+HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE HAKMEM_ALLOC_GATE_SHAPE=1 \
+  ./bench_random_mixed_hakmem 20000000 400 1
+```
+
+### Success Criteria
+
+**GO**: Mean gain >= +1.0%, Median >= +0.0%
+- **Promote to default** in `MIXED_TINYV3_C7_SAFE` preset
+- Add to `core/bench_profile.h`: `bench_setenv_default("HAKMEM_ALLOC_GATE_SHAPE", "1");`
+- Document in `docs/analysis/ENV_PROFILE_PRESETS.md`
+
+**NEUTRAL**: -1.0% < gain < +1.0%
+- **Freeze as research box** (default OFF)
+- Document findings, keep implementation for future study
+
+**NO-GO**: Mean gain <= -1.0%
+- **Freeze and archive** (default OFF, do not pursue)
+- Document regression cause, learn for future optimizations
+
+## 期待値
+
+### Performance Gain Estimation
+
+**Target**: `tiny_alloc_gate_fast` at 12.75% self
+
+**Optimization**:
+- Eliminate policy lookup: ~5-7 cycles saved
+- Add LIKELY hints: ~3-5 cycles saved (branch prediction)
+- Total savings: ~8-12 cycles per allocation
+
+**Calculation**:
+- Baseline: ~100 cycles per allocation (estimated)
+- Savings: 8-12 cycles (8-12% of allocation cost)
+- `tiny_alloc_gate_fast` contribution: 12.75% self
+- Expected gain: 12.75% × (8-12%) = **+1.0-1.5%** (conservative)
+
+**Realistic range**: +1-2% on MIXED workload
+
+### Risk Assessment
+
+**Risk Level**: LOW
+
+**Why**:
+- Only branch shape optimization (no algorithmic change)
+- ENV gate allows instant rollback
+- Fallback path unchanged (safety preserved)
+- Pattern proven by Phase 2 B3 (+2.89% success)
+
+**Failure modes**:
+- Policy lookup cost lower than expected → minimal/no gain
+- ENV check overhead outweighs savings → slight regression
+- Both cases: Rollback with `HAKMEM_ALLOC_GATE_SHAPE=0`
+
+## 非目標
+
+**NOT in scope**:
+- Route algorithm change (only branch shape)
+- Learner integration (optional for future)
+- C6-heavy workload optimization (already uses MID_V3 ON)
+- Policy snapshot bypass (already done in Phase 3 C3)
+
+## Reference Patterns
+
+### Similar Optimizations
+
+**Phase 2 B3** (Routing shape optimization):
+- File: `core/front/malloc_tiny_fast.h:262-278`
+- Pattern: `if (TINY_HOT_LIKELY(route_kind == SMALL_ROUTE_LEGACY))`
+- Result: **+2.89% on MIXED**, +9.13% on C6-heavy
+- Lesson: LIKELY hints + cold helpers are highly effective
+
+**Phase 3 D1** (Free route cache):
+- File: `core/box/tiny_free_route_cache_env_box.h`
+- Pattern: ENV gate with lazy init (-1 sentinel)
+- Result: **+2.19% on MIXED** (promoted to default)
+- Lesson: ENV gates with cached values have minimal overhead
+
+**Phase 3 C3** (Static routing):
+- File: `core/box/tiny_static_route_box.h`
+- Pattern: Static route table (bypass policy snapshot)
+- Result: **+2.20% on MIXED**
+- Lesson: Eliminating dynamic lookups pays off
+
+### Code Examples
+
+**B3 Pattern** (LIKELY-first branching):
+```c
+if (TINY_HOT_LIKELY(env_cfg->alloc_route_shape)) {
+    if (TINY_HOT_LIKELY(route_kind == SMALL_ROUTE_LEGACY)) {
+        // Hot path: LEGACY fast (99% traffic)
+        void* ptr = tiny_hot_alloc_fast(class_idx);
+        if (TINY_HOT_LIKELY(ptr != NULL)) return ptr;
+        return tiny_cold_refill_and_alloc(class_idx);
+    }
+    // Rare routes: cold helper
+    return tiny_alloc_route_cold(route_kind, class_idx, size);
+}
+```
+
+**D1 Pattern** (ENV gate with lazy init):
+```c
+static inline int tiny_free_static_route_enabled(void) {
+    static int g_enable = -1;
+    if (__builtin_expect(g_enable == -1, 0)) {
+        const char* e = getenv("HAKMEM_FREE_STATIC_ROUTE");
+        g_enable = (e && *e && *e != '0') ? 1 : 0;
+    }
+    return g_enable;
+}
+```
+
+## Integration Plan
+
+### Step 1: Implementation
+
+1. Create `core/box/tiny_alloc_gate_shape_env_box.h`
+   - Single function: `alloc_gate_shape_enabled()`
+   - Lazy init with -1 sentinel
+   - Return cached ENV value
+
+2. Modify `core/box/tiny_alloc_gate_box.h`
+   - Add include for new ENV box
+   - Insert LEGACY-first fast path (ENV gated)
+   - Add LIKELY hints to existing branches
+   - Preserve all validation/diagnostic logic
+
+### Step 2: A/B Testing
+
+1. Build with optimization:
+   ```bash
+   make clean && make -j8 CFLAGS="-O3 -flto"
+   ```
+
+2. Run 10-run baseline:
+   ```bash
+   for i in {1..10}; do
+       HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE HAKMEM_ALLOC_GATE_SHAPE=0 \
+         ./bench_random_mixed_hakmem 20000000 400 1
+   done
+   ```
+
+3. Run 10-run optimized:
+   ```bash
+   for i in {1..10}; do
+       HAKMEM_PROFILE=MIXED_TINYV3_C7_SAFE HAKMEM_ALLOC_GATE_SHAPE=1 \
+         ./bench_random_mixed_hakmem 20000000 400 1
+   done
+   ```
+
+4. Calculate statistics:
+   - Mean, Median, StdDev for both configurations
+   - Compare against success criteria
+   - Document results in `PHASE4_D3_ALLOC_GATE_AB_TEST_RESULTS.md`
+
+### Step 3: Decision & Promotion
+
+**If GO**:
+1. Update `core/bench_profile.h` (MIXED_TINYV3_C7_SAFE preset)
+2. Update `docs/analysis/ENV_PROFILE_PRESETS.md`
+3. Update `CURRENT_TASK.md` (Phase 4 D3 complete)
+4. Commit with message: "Phase 4 D3: Alloc Gate Specialization (+X.X%)"
+
+**If NEUTRAL or NO-GO**:
+1. Document results in `PHASE4_D3_ALLOC_GATE_AB_TEST_RESULTS.md`
+2. Keep ENV gate at default OFF
+3. Archive as research box
+4. Move to next candidate optimization
+
+## Validation Checklist
+
+**Pre-implementation**:
+- [ ] Design document reviewed and approved
+- [ ] Integration points identified (2 files)
+- [ ] Reference patterns studied (B3, D1, C3)
+- [ ] ENV gate strategy confirmed (opt-in, default OFF)
+
+**Implementation**:
+- [ ] `tiny_alloc_gate_shape_env_box.h` created
+- [ ] `tiny_alloc_gate_box.h` modified (LEGACY-first path added)
+- [ ] LIKELY hints added to fallback branches
+- [ ] Clean compilation (no new warnings)
+- [ ] Health check passes: `scripts/verify_health_profiles.sh`
+
+**A/B Testing**:
+- [ ] Baseline 10-run completed (SHAPE=0)
+- [ ] Optimized 10-run completed (SHAPE=1)
+- [ ] Statistics calculated (mean, median, stddev)
+- [ ] Results documented
+- [ ] Success criteria evaluated (GO/NEUTRAL/NO-GO)
+
+**Promotion** (if GO):
+- [ ] `bench_profile.h` updated (default SHAPE=1)
+- [ ] `ENV_PROFILE_PRESETS.md` updated
+- [ ] `CURRENT_TASK.md` updated (Phase 4 complete)
+- [ ] Commit created with clear message
+- [ ] Cumulative gain updated (+X.X% total)
+
+---
+
+**Phase 4 D3 Status**: DESIGN COMPLETE
+**Next Step**: Implementation → A/B Test → Decision
+**Expected Outcome**: +1-2% gain (conservative), LOW risk
+**Rollback Plan**: `HAKMEM_ALLOC_GATE_SHAPE=0` (instant revert)