Phase FREE-LEGACY-OPT-6: C4 ULTRA Implementation

Implement C4 ULTRA free TLS cache with parasitic free+alloc pattern,
achieving 99.7-99.9% elimination of C4 legacy fallback calls.

Key Features:
- TLS cache cap=64 (tuned for L1 cache fit, smaller than C5/C6's 128)
- Segment learning via ss_fast_lookup() on first free
- Free-side cache push + alloc-side TLS pop pattern
- ENV gate: HAKMEM_TINY_C4_ULTRA_FREE_ENABLED (default OFF)
- Full FREE_PATH_STATS instrumentation

Benchmark Results:
C4-heavy (65-128B range):
  - C4 legacy: 591,583 → 1,711 (-99.7%)
  - c4_ultra cache hits: ~599k (free) + ~599k (alloc)
  - Mixed load: 340,732 → 284 C4 legacy (-99.9%)

Legacy fallback reduction:
  - C4-heavy: 589,872 fewer legacy calls (-10.9% total)
  - Mixed: 340,448 fewer C4 legacy calls (-12.8% in mixed)

Performance note: ~2% throughput cost in isolated C4-heavy case,
acceptable tradeoff for 99%+ legacy elimination per class.

Files:
  NEW: core/box/tiny_c4_ultra_free_box.h/c
  NEW: core/box/tiny_c4_ultra_free_env_box.h
  MOD: core/box/tiny_ultra_classes_box.h (added C4 macros)
  MOD: core/box/free_path_stats_box.h/c (C4 ULTRA counters)
  MOD: core/front/malloc_tiny_fast.h (C4 alloc+free integration)
  MOD: Makefile (added C4 ULTRA object)

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

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

core/box/free_path_stats_box.h

@@ -13,6 +13,8 @@ typedef struct FreePathStats {
     uint64_t c6_ultra_alloc_hit;   // Phase 4-4: C6 ULTRA-alloc (TLS pop)
     uint64_t c5_ultra_free_fast;   // Phase 5-1: C5 ULTRA-free
     uint64_t c5_ultra_alloc_hit;   // Phase 5-2: C5 ULTRA-alloc (TLS pop)
+    uint64_t c4_ultra_free_fast;   // Phase 6: C4 ULTRA-free (cap=64)
+    uint64_t c4_ultra_alloc_hit;   // Phase 6: C4 ULTRA-alloc (TLS pop)
     uint64_t smallheap_v3_fast;
     uint64_t smallheap_v6_fast;
     uint64_t tiny_heap_v1_fast;