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_hot_box.h

@@ -126,7 +126,7 @@ static inline void* tiny_hot_alloc_fast(int class_idx) {
         TINY_HOT_METRICS_HIT(class_idx);
 
         // Write header + return USER pointer (no branch)
-        #ifdef HAKMEM_TINY_HEADER_CLASSIDX
+        #if HAKMEM_TINY_HEADER_CLASSIDX
         tiny_region_id_write_header(base, class_idx);  // 1-byte header at BASE
         return (void*)((char*)base + 1);  // Return USER pointer (BASE+1)
         #else