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>
