fix(Phase 11+): Cold Start lazy init for unified_cache_refill

Root cause: unified_cache_refill() accessed cache->slots before initialization
when a size class was first used via the refill path (not pop path).

Fix: Add lazy initialization check at start of unified_cache_refill()
- Check if cache->slots is NULL before accessing
- Call unified_cache_init() if needed
- Return NULL if init fails (graceful degradation)

Also includes:
- ss_cold_start_box.inc.h: Box Pattern for default prewarm settings
- hakmem_super_registry.c: Use static array in prewarm (avoid recursion)
- Default prewarm enabled (1 SuperSlab/class, configurable via ENV)

Test: 8B→16B→Mixed allocation pattern now works correctly

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

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

core/front/tiny_unified_cache.c

@@ -303,6 +303,15 @@ void* unified_cache_refill(int class_idx) {
 
     TinyUnifiedCache* cache = &g_unified_cache[class_idx];
 
+    // ✅ Phase 11+: Ensure cache is initialized (lazy init for cold path)
+    if (!cache->slots) {
+        unified_cache_init();
+        // Re-check after init (may fail due to alloc failure)
+        if (!cache->slots) {
+            return NULL;
+        }
+    }
+
     // Step 2: Calculate available room in unified cache
     int room = (int)cache->capacity - 1;  // Leave 1 slot for full detection
     if (cache->head > cache->tail) {