Refactor: Split hakmem_tiny_superslab.c + unified backend exit point

Major refactoring to improve maintainability and debugging:

1. Split hakmem_tiny_superslab.c (1521 lines) into 7 focused files:
   - superslab_allocate.c: SuperSlab allocation/deallocation
   - superslab_backend.c: Backend allocation paths (legacy, shared)
   - superslab_ace.c: ACE (Adaptive Cache Engine) logic
   - superslab_slab.c: Slab initialization and bitmap management
   - superslab_cache.c: LRU cache and prewarm cache management
   - superslab_head.c: SuperSlabHead management and expansion
   - superslab_stats.c: Statistics tracking and debugging

2. Created hakmem_tiny_superslab_internal.h for shared declarations

3. Added superslab_return_block() as single exit point for header writing:
   - All backend allocations now go through this helper
   - Prevents bugs where headers are forgotten in some paths
   - Makes future debugging easier

4. Updated Makefile for new file structure

5. Added header writing to ss_legacy_backend_box.c and
   ss_unified_backend_box.c (though not currently linked)

Note: Header corruption bug in Larson benchmark still exists.
Class 1-6 allocations go through TLS refill/carve paths, not backend.
Further investigation needed.

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

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

core/box/ss_unified_backend_box.c

@@ -7,6 +7,7 @@
 #include "hakmem_shared_pool.h"
 #include "hakmem_tiny_config.h"
 #include "ss_allocation_box.h"
+#include "../tiny_region_id.h"  // For tiny_region_id_write_header
 #include <stdio.h>
 #include <stdlib.h>
 
@@ -109,7 +110,11 @@ void* hak_tiny_alloc_superslab_backend_shared(int class_idx)
 
     hak_tiny_ss_hint_record(class_idx, ss, slab_idx);
 
+#if HAKMEM_TINY_HEADER_CLASSIDX
+    return tiny_region_id_write_header(base, class_idx);
+#else
     return (void*)base;
+#endif
 }
 
 // ============================================================================