Phase 86: Free Path Legacy Mask (NO-GO, +0.25%)

## Summary

Implemented Phase 86 "mask-only commit" optimization for free path:
- Bitset mask (0x7f for C0-C6) to identify LEGACY classes
- Direct call to tiny_legacy_fallback_free_base_with_env()
- No indirect function pointers (avoids Phase 85's -0.86% regression)
- Fail-fast on LARSON_FIX=1 (cross-thread validation incompatibility)

## Results (10-run SSOT)

**NO-GO**: +0.25% improvement (threshold: +1.0%)
- Control:    51,750,467 ops/s (CV: 2.26%)
- Treatment:  51,881,055 ops/s (CV: 2.32%)
- Delta:      +0.25% (mean), -0.15% (median)

## Root Cause

Competing optimizations plateau:
1. Phase 9/10 MONO LEGACY (+1.89%) already capture most free path benefit
2. Remaining margin insufficient to overcome:
   - Two branch checks (mask_enabled + has_class)
   - I-cache layout tax in hot path
   - Direct function call overhead

## Phase 85 vs Phase 86

| Metric | Phase 85 | Phase 86 |
|--------|----------|----------|
| Approach | Indirect calls + table | Bitset mask + direct call |
| Result | -0.86% | +0.25% |
| Verdict | NO-GO (regression) | NO-GO (insufficient) |

Phase 86 correctly avoided indirect call penalties but revealed architectural
limit: can't escape Phase 9/10 overlay without restructuring.

## Recommendation

Free path optimization layer has reached practical ceiling:
- Phase 9/10 +1.89% + Phase 6/19/FASTLANE +16-27% ≈ 18-29% total
- Further attempts on ceremony elimination face same constraints
- Recommend focus on different optimization layers (malloc, etc.)

## Files Changed

### New
- core/box/free_path_legacy_mask_box.h (API + globals)
- core/box/free_path_legacy_mask_box.c (refresh logic)

### Modified
- core/bench_profile.h (added refresh call)
- core/front/malloc_tiny_fast.h (added Phase 86 fast path check)
- Makefile (added object files)
- CURRENT_TASK.md (documented result)

All changes conditional on HAKMEM_FREE_PATH_LEGACY_MASK=1 (default OFF).

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

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

docs/analysis/RESEARCH_BOXES_SSOT.md

@@ -39,3 +39,11 @@
 - 研究箱を“削除”するのは、次の条件を満たしたときだけ:
   - (1) 少なくとも 2週間以上使っていない、(2) SSOT/bench_profile/cleanenv が参照していない、
     (3) 同一バイナリ A/B で削除しても性能が変わらない（layout tax 無い）ことを確認した。
+
+## 外部相談のSSOT（貼り付けパケット）
+
+凍結箱が増えてくると「どの経路を踏んでるか」が外部に説明しづらくなるので、
+レビュー依頼は “圧縮パケット” を正として使う:
+
+- 生成: `scripts/make_chatgpt_pro_packet_free_path.sh`
+- スナップショット: `docs/analysis/FREE_PATH_REVIEW_PACKET_CHATGPT.md`