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/front_gate_classifier.c

@@ -80,6 +80,7 @@ static inline int safe_header_probe(void* ptr) {
         return -1;
     }
 
+#if HAKMEM_TINY_HEADER_CLASSIDX
     // Safe to read header (same page guaranteed)
     uint8_t* header_ptr = (uint8_t*)ptr - 1;
     uint8_t header = *header_ptr;
@@ -98,6 +99,9 @@ static inline int safe_header_probe(void* ptr) {
     }
 
     return class_idx;
+#else
+    return -1;
+#endif
 }
 
 // ========== Registry Lookup ==========
@@ -200,6 +204,7 @@ ptr_classification_t classify_ptr(void* ptr) {
         uint8_t magic = header & 0xF0;
 
         // Fast path: Tiny allocation (magic = 0xa0) — guarded by Superslab registry
+#if HAKMEM_TINY_HEADER_CLASSIDX
         if (magic == HEADER_MAGIC) {  // HEADER_MAGIC = 0xa0
             int class_idx = header & HEADER_CLASS_MASK;
             if (class_idx >= 0 && class_idx < TINY_NUM_CLASSES) {
@@ -219,6 +224,7 @@ ptr_classification_t classify_ptr(void* ptr) {
                 }
             }
         }
+#endif
 
 #ifdef HAKMEM_POOL_TLS_PHASE1
         // Fast path: Pool TLS allocation (magic = 0xb0)