Phase FREE-LEGACY-OPT-5-1/5-2: C5 ULTRA free+alloc integration

Summary:
========
Implemented C5 ULTRA TLS cache pattern following the successful C6 ULTRA design:
- Phase 5-1: Free-side TLS cache + segment learning
- Phase 5-2: Alloc-side TLS pop for complete free+alloc cycle integration

Targets C5 class (129-256B) as next legacy reduction after C6 completion.

Key Changes:
============

1. NEW FILES:
   - core/box/tiny_c5_ultra_free_box.h: C5 ULTRA TLS cache structure
   - core/box/tiny_c5_ultra_free_box.c: C5 free path implementation (same pattern as C6)
   - core/box/tiny_c5_ultra_free_env_box.h: ENV gating (HAKMEM_TINY_C5_ULTRA_FREE_ENABLED)

2. MODIFIED FILES:
   - core/front/malloc_tiny_fast.h:
     * Added C5 ULTRA includes
     * Added C5 alloc-side TLS pop at lines 186-194 (integrated with C6)
     * Added C5 free path at lines 333-337 (integrated with C6)

   - core/box/tiny_ultra_classes_box.h:
     * Added TINY_CLASS_C5 constant
     * Added tiny_class_is_c5() macro
     * Extended tiny_class_is_ultra() to include C5

   - core/box/free_path_stats_box.h:
     * Added c5_ultra_free_fast counter
     * Added c5_ultra_alloc_hit counter

   - core/box/free_path_stats_box.c:
     * Updated stats dump to output C5 counters

   - Makefile:
     * Added core/box/tiny_c5_ultra_free_box.o to all object lists

3. Design Rationale:
   - Exact copy of C6 ULTRA pattern (proven effective)
   - TLS cache capacity: 128 blocks (same as C6 for consistency)
   - Segment learning on first C5 free via ss_fast_lookup()
   - Alloc-side pop integrated directly in malloc_tiny_fast.h hotpath
   - Legacy fallback unification via tiny_legacy_fallback_free_base()

4. Expected Impact:
   - C5 legacy calls: 68,871 → 0 (100% elimination)
   - Total legacy reduction: ~53% of remaining 129,623
   - Mixed workload: Minimal regression (C5 is smaller class, fewer allocations)

5. Stats Collection:
   Run with: HAKMEM_TINY_C5_ULTRA_FREE_ENABLED=1 HAKMEM_FREE_PATH_STATS=1 ./bench_allocators_hakmem

   Expected output:
   [FREE_PATH_STATS] ... c5_ultra_free=68871 c5_ultra_alloc=68871 ... legacy_fb=60752 ...
   [FREE_PATH_STATS_LEGACY_BY_CLASS] ... c5=0 ...

Status:
=======
- Code: ✅ COMPLETE (3 new files + 5 modified files)
- Compilation: ✅ Verified (no errors, only unused variable warnings unrelated to C5)
- Functionality: Ready to benchmark (ENV gating: default OFF, opt-in via ENV)

Phase Progression:
==================
✅ Phase 4-4: C6 ULTRA free+alloc (legacy C6: 137,319 → 0)
✅ Phase 5-1/5-2: C5 ULTRA free+alloc (legacy C5: 68,871 → 0 expected)
⏳ Phase 4.5: C4 ULTRA (34,727 remaining)
📋 Future: C3/C2 ULTRA if beneficial

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

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

core/front/malloc_tiny_fast.h

@@ -47,6 +47,7 @@
 // #include "../box/smallobject_core_v6_box.h" // SmallObject Core v6 (C6-only route stub, Phase v6-1)
 #include "../box/tiny_c7_ultra_box.h" // C7 ULTRA stub (UF-1, delegates to v3)
 #include "../box/tiny_c6_ultra_free_box.h" // Phase 4-2: C6 ULTRA-free (free-only, C6-only)
+#include "../box/tiny_c5_ultra_free_box.h" // Phase 5-1/5-2: C5 ULTRA-free + alloc integration
 #include "../box/tiny_ultra_classes_box.h" // Phase REFACTOR-1: Named constants for C6/C7
 #include "../box/tiny_legacy_fallback_box.h" // Phase REFACTOR-2: Legacy fallback logic unification
 #include "../box/tiny_ptr_convert_box.h" // Phase REFACTOR-3: Inline pointer macro centralization
@@ -182,6 +183,16 @@ static inline void* malloc_tiny_fast(size_t size) {
         }
     }
 
+    // Phase 5-2: C5 ULTRA free+alloc integration (same pattern as C6)
+    if (tiny_class_is_c5(class_idx) && tiny_c5_ultra_free_enabled()) {
+        TinyC5UltraFreeTLS* ctx = tiny_c5_ultra_free_tls();
+        if (TINY_HOT_LIKELY(ctx->count > 0)) {
+            void* base = ctx->freelist[--ctx->count];
+            FREE_PATH_STAT_INC(c5_ultra_alloc_hit);
+            return tiny_base_to_user_inline(base);
+        }
+    }
+
     switch (route) {
         case TINY_ROUTE_SMALL_HEAP_V6: {
             // Phase FREE-LEGACY-BREAKDOWN-1: v6 は既存のビルドエラーがあるため一時的にスキップ
@@ -319,6 +330,12 @@ static inline int free_tiny_fast(void* ptr) {
         return 1;
     }
 
+    // Phase 5-1: C5 ULTRA-free (C5-only, free-only, ENV gated, same pattern as C6)
+    if (tiny_class_is_c5(class_idx) && tiny_c5_ultra_free_enabled()) {
+        tiny_c5_ultra_free_fast(base, class_idx);
+        return 1;
+    }
+
     // C7 v3 fast classify: bypass classify_ptr/ss_map_lookup for clear hits
     if (class_idx == 7 &&
         tiny_front_v3_enabled() &&