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/tiny_refill_opt.h

@@ -6,6 +6,7 @@
 #include <stdio.h>
 #include <stdatomic.h>
 #include <stdlib.h>
+#include "hakmem_build_flags.h" // Ensure flags are overridden
 #include "tiny_region_id.h"  // For HEADER_MAGIC, HEADER_CLASS_MASK (Fix #6)
 #include "ptr_track.h"        // Pointer tracking for debugging header corruption
 #include "box/tiny_next_ptr_box.h"  // Box API: Next pointer read/write
@@ -251,7 +252,7 @@ static inline uint32_t trc_pop_from_freelist(struct TinySlabMeta* meta,
         // DEBUG: Log header restoration for class 2
         uint8_t before = *(uint8_t*)p;
         PTR_TRACK_FREELIST_POP(p, class_idx);
-        *(uint8_t*)p = HEADER_MAGIC | (class_idx & HEADER_CLASS_MASK);
+        tiny_header_write_if_preserved(p, class_idx); // Box API
         PTR_TRACK_HEADER_WRITE(p, HEADER_MAGIC | (class_idx & HEADER_CLASS_MASK));
         static _Atomic uint64_t g_freelist_count_c2 = 0;
         if (class_idx == 2) {
@@ -320,7 +321,7 @@ static inline uint32_t trc_linear_carve(uint8_t* base, size_t bs,
     for (uint32_t i = 0; i < batch; i++) {
         uint8_t* block = cursor + (i * stride);
         PTR_TRACK_CARVE((void*)block, class_idx);
-        *block = HEADER_MAGIC | (class_idx & HEADER_CLASS_MASK);
+        tiny_header_write_if_preserved((void*)block, class_idx); // Box API
         PTR_TRACK_HEADER_WRITE((void*)block, HEADER_MAGIC | (class_idx & HEADER_CLASS_MASK));
 
 #if !HAKMEM_BUILD_RELEASE