Phase v4-mid-1: C6-only v4 route + page_meta_of() Fail-Fast validation

Implementation:
- SMALL_SEGMENT_V4_* constants (SIZE=2MiB, PAGE_SIZE=64KiB, MAGIC=0xDEADBEEF)
- smallsegment_v4_page_meta_of(): O(1) mask+shift lookup with magic validation
  - Computes segment base: addr & ~(2MiB - 1)
  - Verifies SmallSegment magic number
  - Calculates page_idx: (addr - seg_base) >> PAGE_SHIFT (16)
  - Returns non-NULL sentinel for now (full page_meta[] in Phase v4-mid-2)

Stubs for C6-only phase:
- small_heap_alloc_fast_v4(): C6 returns NULL → pool v1 fallback
- small_heap_free_fast_v4(): C6 calls page_meta_of() for Fail-Fast, then pool v1 fallback

Documentation:
- ENV_PROFILE_PRESETS.md: Add "C6_ONLY_SMALLOBJECT_V4" research profile
  - HAKMEM_SMALL_HEAP_V4_ENABLED=1, HAKMEM_SMALL_HEAP_V4_CLASSES=0x40
  - Expected: Throughput ≈ 28–29M ops/s (same as v1)

Build:
- ビルド成功（警告のみ）
- Backward compatible, alloc/free stubs fall back to pool v1

Sanity:
- C6-heavy with v4 opt-in: segv/assert なし
- page_meta_of() lookup working correctly
- Performance unchanged (expected for stub phase)

Status:
- C6-only v4 route now available via ENV opt-in
- Phase v4-mid-2: SmallHeapCtx v4 full implementation with A/B

🤖 Generated with Claude Code

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/analysis/ENV_PROFILE_PRESETS.md

@@ -135,6 +135,63 @@ HAKMEM_BENCH_MAX_SIZE=768
 
 ---
 
+## Research Profile 1: C6_ONLY_SMALLOBJECT_V4（SmallObject v4 C6-only 試運転）
+
+### 目的
+- C6-only を SmallObject v4 route に載せて、page_meta_of() の試運転。
+- 挙動はまだ pool v1 fallback のため、perf は v1 固定と同じ。
+- Phase v4-mid-1: page_meta_of() が落ちないか、segv/assert なしか確認する研究ベンチ。
+
+### ENV（v4 C6-only opt-in）
+```sh
+HAKMEM_BENCH_MIN_SIZE=257
+HAKMEM_BENCH_MAX_SIZE=768
+HAKMEM_TINY_HEAP_PROFILE=C7_SAFE
+HAKMEM_TINY_C6_HOT=0
+HAKMEM_TINY_HOTHEAP_V2=0
+HAKMEM_SMALL_HEAP_V3_ENABLED=1
+HAKMEM_SMALL_HEAP_V3_CLASSES=0x80        # C7-only v3（C6 は v3 OFF）
+HAKMEM_SMALL_HEAP_V4_ENABLED=1           # ★ v4 ON
+HAKMEM_SMALL_HEAP_V4_CLASSES=0x40        # ★ C6(bit6) だけ v4 route に
+HAKMEM_POOL_V2_ENABLED=0
+HAKMEM_POOL_V1_FLATTEN_ENABLED=0
+```
+
+### テストコマンド
+```sh
+export HAKMEM_PROFILE=C6_HEAVY_LEGACY_POOLV1
+export HAKMEM_SMALL_HEAP_V4_ENABLED=1
+export HAKMEM_SMALL_HEAP_V4_CLASSES=0x40
+./bench_mid_large_mt_hakmem 1000000 400 1
+```
+
+### 期待値
+- Throughput ≈ **28–29M ops/s**（v1 基線の ≈28M と同じ）
+- segv/assert なし
+- small_segment_v4_page_meta_of(ptr) が動く（debug output で確認可能）
+
+### 注意
+- 実際の alloc/free 動作は pool v1 のまま（v4 freelist は使わない）
+- Phase v4-mid-2 で本格実装時に差し替える
+
+---
+
+## Research Profile 2: C7_C6_V4_EXPERIMENT（C7+C6 v4 統合研究）
+
+### 目的
+- 後続フェーズで C7+C6 両者を v4 に載せるときの参考プリセット。
+- 現フェーズではまだ使わない（v4-mid-1 は C6-only）。
+
+### ENV（参考用）
+```sh
+HAKMEM_BENCH_MIN_SIZE=16
+HAKMEM_BENCH_MAX_SIZE=1024
+HAKMEM_SMALL_HEAP_V4_ENABLED=1
+HAKMEM_SMALL_HEAP_V4_CLASSES=0xC0        # C6(0x40) + C7(0x80)
+```
+
+---
+
 ## Profile 3: DEBUG_TINY_FRONT_PERF（perf 用 DEBUG プロファイル）
 
 ### 目的