Phase 13-B: TinyHeapV2 free path supply hook (magazine population)

Implement magazine supply from free path to enable TinyHeapV2 L0 cache

Changes:
1. core/tiny_free_fast_v2.inc.h (Line 24, 134-143):
   - Include tiny_heap_v2.h for magazine API
   - Add supply hook after BASE pointer conversion (Line 134-143)
   - Try to push freed block to TinyHeapV2 magazine (C0-C3 only)
   - Falls back to TLS SLL if magazine full (existing behavior)

2. core/front/tiny_heap_v2.h (Line 24-46):
   - Move TinyHeapV2Mag / TinyHeapV2Stats typedef from hakmem_tiny.c
   - Add extern declarations for TLS variables
   - Define TINY_HEAP_V2_MAG_CAP (16 slots)
   - Enables use from tiny_free_fast_v2.inc.h

3. core/hakmem_tiny.c (Line 1270-1276, 1766-1768):
   - Remove duplicate typedef definitions
   - Move TLS storage declarations after tiny_heap_v2.h include
   - Reason: tiny_heap_v2.h must be included AFTER tiny_alloc_fast.inc.h
   - Forward declarations remain for early reference

Supply Hook Flow:
```
hak_free_at(ptr) → hak_tiny_free_fast_v2(ptr)
  → class_idx = read_header(ptr)
  → base = ptr - 1
  → if (class_idx <= 3 && tiny_heap_v2_enabled())
      → tiny_heap_v2_try_push(class_idx, base)
        → success: return (magazine supplied)
        → full: fall through to TLS SLL
  → tls_sll_push(class_idx, base)  # existing path
```

Benefits:
- Magazine gets populated from freed blocks (L0 cache warm-up)
- Next allocation hits magazine (fast L0 path, no backend refill)
- Expected: 70-90% hit rate for fixed-size workloads
- Expected: +200-500% performance for C0-C3 classes

Build & Smoke Test:
- ✅ Build successful
- ✅ bench_fixed_size 256B workset=50: 33M ops/s (stable)
- ✅ bench_fixed_size 16B workset=60: 30M ops/s (stable)
- 🔜 A/B test (hit rate measurement) deferred to next commit

Implementation Status:
- ✅ Phase 13-A: Alloc hook + stats (completed, committed)
- ✅ Phase 13-B: Free path supply (THIS COMMIT)
- 🔜 Phase 13-C: Evaluation & tuning

Notes:
- Supply hook is C0-C3 only (TinyHeapV2 target range)
- Magazine capacity=16 (same as Phase 13-A)
- No performance regression (hook is ENV-gated: HAKMEM_TINY_HEAP_V2=1)

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

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

core/front/tiny_heap_v2.h

@@ -21,8 +21,29 @@
 
 #include "../hakmem_tiny.h"
 
-// NOTE: TinyHeapV2Mag struct and g_tiny_heap_v2_mag are defined in hakmem_tiny.c
-// This header provides only the implementations (static inline functions).
+// Phase 13-B: Magazine capacity (same as Phase 13-A)
+#ifndef TINY_HEAP_V2_MAG_CAP
+#define TINY_HEAP_V2_MAG_CAP 16
+#endif
+
+// TinyHeapV2 Magazine (per-thread, per-class)
+typedef struct {
+    void* items[TINY_HEAP_V2_MAG_CAP];
+    int   top;
+} TinyHeapV2Mag;
+
+// TinyHeapV2 Statistics (per-thread, per-class)
+typedef struct {
+    uint64_t alloc_calls;
+    uint64_t mag_hits;
+    uint64_t refill_calls;
+    uint64_t refill_blocks;
+    uint64_t backend_oom;
+} TinyHeapV2Stats;
+
+// External TLS variables (defined in hakmem_tiny.c)
+extern __thread TinyHeapV2Mag g_tiny_heap_v2_mag[TINY_NUM_CLASSES];
+extern __thread TinyHeapV2Stats g_tiny_heap_v2_stats[TINY_NUM_CLASSES];
 
 // Enable flag (cached)
 static inline int tiny_heap_v2_enabled(void) {