Phase 15 v1: UnifiedCache FIFO→LIFO NEUTRAL (-0.70% Mixed, +0.42% C7)

Transform existing array-based UnifiedCache from FIFO ring to LIFO stack.

A/B Results:
- Mixed (16-1024B): -0.70% (52,965,966 → 52,593,948 ops/s)
- C7-only (1025-2048B): +0.42% (78,010,783 → 78,335,509 ops/s)

Verdict: NEUTRAL (both below +1.0% GO threshold) - freeze as research box

Implementation:
- L0 ENV gate: tiny_unified_lifo_env_box.{h,c} (HAKMEM_TINY_UNIFIED_LIFO=0/1)
- L1 LIFO ops: tiny_unified_lifo_box.h (unified_cache_try_pop/push_lifo)
- L2 integration: tiny_front_hot_box.h (mode check at entry)
- Reuses existing slots[] array (no intrusive pointers)

Root Causes:
1. Mode check overhead (tiny_unified_lifo_enabled() call)
2. Minimal LIFO vs FIFO locality delta in practice
3. Existing FIFO ring already well-optimized

Bonus Fix: LTO bug for tiny_c7_preserve_header_enabled() (Phase 13/14 latent issue)
- Converted static inline to extern + non-inline implementation
- Fixes undefined reference during LTO linking

Design: docs/analysis/PHASE15_UNIFIEDCACHE_LIFO_1_DESIGN.md
Results: docs/analysis/PHASE15_UNIFIEDCACHE_LIFO_1_AB_TEST_RESULTS.md

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

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

core/bench_profile.h

@@ -12,6 +12,7 @@
 #include "box/tiny_free_route_cache_env_box.h"  // tiny_free_static_route_refresh_from_env (Phase 8)
 #include "box/tiny_c7_preserve_header_env_box.h"  // tiny_c7_preserve_header_env_refresh_from_env (Phase 13 v1)
 #include "box/tiny_tcache_env_box.h"  // tiny_tcache_env_refresh_from_env (Phase 14 v1)
+#include "box/tiny_unified_lifo_env_box.h"  // tiny_unified_lifo_env_refresh_from_env (Phase 15 v1)
 #endif
 
 // env が未設定のときだけ既定値を入れる
@@ -190,5 +191,7 @@ static inline void bench_apply_profile(void) {
 	  tiny_c7_preserve_header_env_refresh_from_env();
 	  // Phase 14 v1: Sync tcache ENV cache after bench_profile putenv defaults.
 	  tiny_tcache_env_refresh_from_env();
+	  // Phase 15 v1: Sync LIFO ENV cache after bench_profile putenv defaults.
+	  tiny_unified_lifo_env_refresh_from_env();
 #endif
 	}