Implement Phase 2: Headerless Allocator Support (Partial)

- Feature: Added HAKMEM_TINY_HEADERLESS toggle (A/B testing)
- Feature: Implemented Headerless layout logic (Offset=0)
- Refactor: Centralized layout definitions in tiny_layout_box.h
- Refactor: Abstracted pointer arithmetic in free path via ptr_conversion_box.h
- Verification: sh8bench passes in Headerless mode (No TLS_SLL_HDR_RESET)
- Known Issue: Regression in Phase 1 mode due to blind pointer conversion logic

core/box/tiny_front_cold_box.h

@@ -75,7 +75,7 @@ static inline void* tiny_cold_refill_and_alloc(int class_idx) {
     // Success: return USER pointer
     // NOTE: Header already written by unified_cache_refill()
     // (Removed redundant tiny_region_id_write_header() - P2 fix)
-    #ifdef HAKMEM_TINY_HEADER_CLASSIDX
+    #if HAKMEM_TINY_HEADER_CLASSIDX
     return (void*)((char*)base + 1);  // USER pointer
     #else
     return base;