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/ptr_conversion_box.h

@@ -44,8 +44,15 @@ static inline hak_user_ptr_t ptr_base_to_user(hak_base_ptr_t base, int class_idx
         return HAK_USER_FROM_RAW(NULL);
     }
 
+#if HAKMEM_TINY_HEADERLESS
+    (void)class_idx;
+    // Headerless: user = base (identity)
+    void* raw_user = raw_base;
+    size_t offset = 0;
+#else
     size_t offset = tiny_user_offset(class_idx);
     void* raw_user = (void*)((uint8_t*)raw_base + offset);
+#endif
     
     PTR_CONV_LOG("BASE→USER cls=%d base=%p → user=%p (+%zu)\n",
                  class_idx, raw_base, raw_user, offset);
@@ -66,14 +73,37 @@ static inline hak_base_ptr_t ptr_user_to_base(hak_user_ptr_t user, int class_idx
         return HAK_BASE_FROM_RAW(NULL);
     }
 
+#if HAKMEM_TINY_HEADERLESS
+    (void)class_idx;
+    // Headerless: base = user (identity)
+    void* raw_base = raw_user;
+    size_t offset = 0;
+#else
     size_t offset = tiny_user_offset(class_idx);
     void* raw_base = (void*)((uint8_t*)raw_user - offset);
+#endif
     
     PTR_CONV_LOG("USER→BASE cls=%d user=%p → base=%p (-%zu)\n",
                  class_idx, raw_user, raw_base, offset);
     return HAK_BASE_FROM_RAW(raw_base);
 }
 
+/**
+ * Convert USER pointer to BASE pointer WITHOUT knowing class_idx
+ * Primary use: free() entry point where class is not yet known
+ */
+static inline void* ptr_user_to_base_blind(void* user_ptr) {
+    if (user_ptr == NULL) return NULL;
+
+#if HAKMEM_TINY_HEADERLESS
+    // Headerless: base = user
+    return user_ptr;
+#else
+    // Phase 1: All classes have 1 byte header -> base = user - 1
+    return (void*)((uint8_t*)user_ptr - 1);
+#endif
+}
+
 /**
  * Convenience macros for cleaner call sites
  */