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>

core/bench_profile.h

@@ -17,6 +17,8 @@
 #include "box/fastlane_direct_env_box.h"  // fastlane_direct_env_refresh_from_env (Phase 19-1)
 #include "box/tiny_header_hotfull_env_box.h"  // tiny_header_hotfull_env_refresh_from_env (Phase 21)
 #include "box/tiny_inline_slots_fixed_mode_box.h"  // tiny_inline_slots_fixed_mode_refresh_from_env (Phase 78-1)
+#include "box/free_path_commit_once_fixed_box.h"  // free_path_commit_once_refresh_from_env (Phase 85)
+#include "box/free_path_legacy_mask_box.h"  // free_path_legacy_mask_refresh_from_env (Phase 86)
 #endif
 
 // env が未設定のときだけ既定値を入れる
@@ -235,5 +237,9 @@ static inline void bench_apply_profile(void) {
 		  tiny_header_hotfull_env_refresh_from_env();
 		  // Phase 78-1: Optionally pin C3/C4/C5/C6 inline-slots modes (avoid per-op ENV gates).
 		  tiny_inline_slots_fixed_mode_refresh_from_env();
+		  // Phase 85: Optionally commit-once for C4-C7 LEGACY free path (skip policy/route/mono ceremony).
+		  free_path_commit_once_refresh_from_env();
+		  // Phase 86: Optionally use legacy mask for early exit (no indirect calls, just bit test).
+		  free_path_legacy_mask_refresh_from_env();
 #endif
 		}