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_nextptr.h

@@ -87,7 +87,10 @@ static inline __attribute__((always_inline)) void* tiny_next_load(const void* ba
 static inline __attribute__((always_inline)) void tiny_next_store(void* base, int class_idx, void* next) {
     size_t off = tiny_next_off(class_idx);
 
-#if HAKMEM_TINY_HEADER_CLASSIDX
+#if HAKMEM_TINY_HEADERLESS
+    // Headerless mode: never restore header
+    (void)class_idx;
+#elif HAKMEM_TINY_HEADER_CLASSIDX
     // P2.3: Skip header restoration by default (class_map is now default for class_idx lookup)
     // ENV: HAKMEM_TINY_RESTORE_HEADER=1 to force header restoration (legacy fallback mode)
     if (off != 0) {