Phase 13-B: TinyHeapV2 supply path with dual-mode A/B framework (Stealing vs Leftover)

Summary:
- Implemented free path supply with ENV-gated A/B modes (HAKMEM_TINY_HEAP_V2_LEFTOVER_MODE)
- Mode 0 (Stealing, default): L0 gets freed blocks first → +18% @ 32B
- Mode 1 (Leftover): L1 primary owner, L0 gets leftovers → Box-clean but -5% @ 16B
- Decision: Default to Stealing for performance (ChatGPT analysis: L0 doesn't corrupt learning layer signals)

Performance (100K iterations, workset=128):
- 16B: 43.9M → 45.6M ops/s (+3.9%)
- 32B: 41.9M → 49.6M ops/s (+18.4%) ✅
- 64B: 51.2M → 51.5M ops/s (+0.6%)
- 100% magazine hit rate (supply from free path working correctly)

Implementation:
- tiny_free_fast_v2.inc.h: Dual-mode supply (lines 134-166)
- tiny_heap_v2.h: Add tiny_heap_v2_leftover_mode() flag + rationale doc
- tiny_alloc_fast.inc.h: Alloc hook with tiny_heap_v2_alloc_by_class()
- CURRENT_TASK.md: Updated Phase 13-B status (complete) with A/B results

ENV flags:
- HAKMEM_TINY_HEAP_V2=1                      # Enable TinyHeapV2
- HAKMEM_TINY_HEAP_V2_LEFTOVER_MODE=0        # Mode 0 (Stealing, default)
- HAKMEM_TINY_HEAP_V2_CLASS_MASK=0xE         # C1-C3 only (skip C0 -5% regression)
- HAKMEM_TINY_HEAP_V2_STATS=1                # Print statistics

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

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

core/tiny_alloc_fast.inc.h

@@ -28,6 +28,7 @@
 #include "hakmem_tiny_integrity.h"     // PRIORITY 1-4: Corruption detection
 #ifdef HAKMEM_TINY_HEADER_CLASSIDX
 #include "front/tiny_front_c23.h"      // Phase B: Ultra-simple C2/C3 front
+#include "front/tiny_heap_v2.h"        // Phase 13-A: TinyHeapV2 magazine front
 #endif
 #include <stdio.h>
 
@@ -601,6 +602,17 @@ static inline void* tiny_alloc_fast(size_t size) {
     }
 #endif
 
+    // Phase 13-A: TinyHeapV2 (per-thread magazine, experimental)
+    // ENV-gated: HAKMEM_TINY_HEAP_V2=1
+    // Targets class 0-3 (8-64B) only, falls back to existing path if NULL
+    // PERF: Pass class_idx directly to avoid redundant size→class conversion
+    if (__builtin_expect(tiny_heap_v2_enabled(), 0) && class_idx <= 3) {
+        void* base = tiny_heap_v2_alloc_by_class(class_idx);
+        if (base) {
+            HAK_RET_ALLOC(class_idx, base);  // Header write + return USER pointer
+        }
+    }
+
     // NEW: Front-Direct/SLL-OFF bypass control (TLS cached, lazy init)
     static __thread int s_front_direct_alloc = -1;
     if (__builtin_expect(s_front_direct_alloc == -1, 0)) {