Phase POLICY-FAST-PATH-V2 complete + MID-V35-HOTPATH-OPT-1 design

## Phase POLICY-FAST-PATH-V2 (FROZEN)
- Implementation complete: free_policy_fast_v2_box.h + malloc_tiny_fast.h integration
- A/B Results:
  - Mixed (ws=400): -1.6% regression ❌ (branch cost > skip benefit)
  - C6-heavy (ws=200): +5.4% improvement ✅
- Decision: Default OFF, FROZEN (ws<300 / C6-heavy research only)
- Learning: Large WS causes branch misprediction to dominate

## Phase 3-GRADUATE + ENV probe fix
- 64-probe retry for getenv() stability during bench_profile putenv()
- C6 ULTRA intrusive freelist: FROZEN (research box)

## Phase MID-V35-HOTPATH-OPT-1-DESIGN
- Design doc for next optimization target
- Target: MID v3.5 alloc/free hot path (C5-C6)
- Boxes: Stats Gate, TLS Layout, Boundary Check elimination
- Expected: +3-9% on Mixed mainline

Files:
- core/box/free_policy_fast_v2_box.h (new)
- core/box/free_path_stats_box.h/c (policy_fast_v2_skip counter)
- core/front/malloc_tiny_fast.h (fast-path integration)
- docs/analysis/MID_V35_HOTPATH_OPT_1_DESIGN.md (new)
- docs/analysis/PHASE_3_GRADUATE_*.md (new)
- CURRENT_TASK.md (phase status update)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

core/box/free_path_stats_box.h

@@ -29,16 +29,31 @@ typedef struct FreePathStats {
 
     // Phase 4-1: Legacy per-class breakdown
     uint64_t legacy_by_class[8];  // C0-C7 の Legacy fallback 内訳
+
+    // Phase POLICY-FAST-PATH-V2: Fast-path policy skip
+    uint64_t policy_fast_v2_skip;  // Phase POLICY-FAST-PATH-V2 fast-path skips
 } FreePathStats;
 
 // ENV gate
 static inline bool free_path_stats_enabled(void) {
-    static int g_enabled = -1;
-    if (__builtin_expect(g_enabled == -1, 0)) {
-        const char* e = getenv("HAKMEM_FREE_PATH_STATS");
-        g_enabled = (e && *e && *e != '0') ? 1 : 0;
+    static int g_enabled = -1;     // -1: unknown, 0: off, 1: on
+    static int g_probe_left = 64;  // tolerate early getenv() instability (bench_profile putenv)
+
+    if (__builtin_expect(g_enabled == 1, 1)) return true;
+    if (__builtin_expect(g_enabled == 0, 1)) return false;
+
+    const char* e = getenv("HAKMEM_FREE_PATH_STATS");
+    if (e && *e) {
+        g_enabled = (*e != '0') ? 1 : 0;
+        return g_enabled == 1;
     }
-    return g_enabled;
+
+    if (g_probe_left-- > 0) {
+        return false;  // keep g_enabled==-1, retry later
+    }
+
+    g_enabled = 0;
+    return false;
 }
 
 // Global stats instance