Phase 13-A Step 1 COMPLETE: TinyHeapV2 alloc hook + stats + supply infrastructure

Phase 13-A Status: ✅ COMPLETE
- Alloc hook working (hak_tiny_alloc via hakmem_tiny_alloc_new.inc)
- Statistics accurate (alloc_calls, mag_hits tracked correctly)
- NO-REFILL L0 cache stable (zero performance overhead)
- A/B tests: C1 +0.76%, C2 +0.42%, C3 -0.26% (all within noise)

Changes:
- Added tiny_heap_v2_try_push() infrastructure for Phase 13-B (free path supply)
- Currently unused but provides clean API for magazine supply from free path

Verification:
- Modified bench_fixed_size.c to use hak_alloc_at/hak_free_at (HAKMEM routing)
- Verified HAKMEM routing works: workset=10-127 ✅
- Found separate bug: workset=128 hangs (power-of-2 edge case, not HeapV2 related)

Phase 13-B: Free path supply deferred
- Actual free path: hak_free_at → hak_tiny_free_fast_v2
- Not tiny_free_fast (wrapper-only path)
- Requires hak_tiny_free_fast_v2 integration work

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

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

core/front/tiny_heap_v2.h

@@ -79,6 +79,41 @@ static inline int tiny_heap_v2_refill_mag(int class_idx) {
     return 0;
 }
 
+// Phase 13-A Step 2: Try to push a block into TinyHeapV2 magazine
+// Called from free path to supply magazine with "leftover" blocks.
+// Returns: 1 if pushed successfully, 0 if magazine is full
+static inline int tiny_heap_v2_try_push(int class_idx, void* base) {
+    // 1. Check if class is enabled
+    if (class_idx < 0 || class_idx > 3) return 0;
+    if (!tiny_heap_v2_class_enabled(class_idx)) return 0;
+
+    TinyHeapV2Mag* mag = &g_tiny_heap_v2_mag[class_idx];
+
+    // 2. Check if magazine has room
+    if (mag->top >= TINY_HEAP_V2_MAG_CAP) {
+        return 0;  // Magazine full
+    }
+
+    // 3. Push BASE pointer into magazine
+    mag->items[mag->top++] = base;
+
+    // DEBUG: Log push events
+    static int g_push_dbg = -1;
+    if (g_push_dbg == -1) {
+        const char* e = getenv("HAKMEM_TINY_HEAP_V2_DEBUG");
+        g_push_dbg = (e && *e && *e != '0') ? 1 : 0;
+    }
+    if (g_push_dbg) {
+        static __thread int g_push_count[TINY_NUM_CLASSES] = {0};
+        if (g_push_count[class_idx] < 5) {
+            fprintf(stderr, "[HeapV2-PUSH] C%d push #%d, base=%p, mag->top=%d\n",
+                    class_idx, g_push_count[class_idx]++, base, mag->top);
+        }
+    }
+
+    return 1;  // Success
+}
+
 // Tiny heap v2 alloc – returns BASE pointer or NULL.
 // Phase 13-A Step 1: Minimal "lucky hit" L0 cache (NO REFILL)
 // Strategy: Pop from magazine if available, else return NULL immediately.