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_layout_box.h

@@ -17,11 +17,32 @@
 #include <stddef.h>
 #include "../hakmem_tiny_config.h" // For g_tiny_class_sizes and TINY_NUM_CLASSES
 
+// A/B Toggle: Headerless mode
+// ENV: HAKMEM_TINY_HEADERLESS=1 to enable
+// Default: 0 (Phase 1 compatible)
+#ifndef HAKMEM_TINY_HEADERLESS
+  #define HAKMEM_TINY_HEADERLESS 0
+#endif
+
 // Define all class-specific layout parameters
 // Current: Defined in g_tiny_class_sizes[8] in hakmem_tiny.c
 // This file makes them accessible via a unified Box API
 
-// Header size is 1 byte when enabled
+// Header size
+static inline size_t tiny_header_size(int class_idx) {
+#if HAKMEM_TINY_HEADERLESS
+    (void)class_idx;
+    return 0;
+#else
+    // Phase 1: 1 byte header if enabled
+    // C0 (8B): offset 0 (8B stride too small for header + 8B pointer - would overflow)
+    // C7 (2048B): offset 0 (overwrites header in freelist - largest class can tolerate)
+    // C1-C6: offset 1 (header preserved - user data is not disturbed)
+    return (0x7Eu >> class_idx) & 1u;
+#endif
+}
+
+// Legacy macro for backward compatibility (Phase 1)
 #define TINY_HEADER_SIZE 1
 
 // Validation macros
@@ -38,7 +59,10 @@ static inline size_t tiny_class_stride(int class_idx) {
 // Calculate user pointer offset from base pointer
 // This logic centralizes the "User = Base + 1" vs "User = Base + 0" decision
 static inline size_t tiny_user_offset(int class_idx) {
-#if HAKMEM_TINY_HEADER_CLASSIDX
+#if HAKMEM_TINY_HEADERLESS
+    (void)class_idx;
+    return 0;  // Headerless: user = base
+#elif HAKMEM_TINY_HEADER_CLASSIDX
     // C0 (8B): offset 0 (8B stride too small for header + 8B pointer - would overflow)
     // C7 (2048B): offset 0 (overwrites header in freelist - largest class can tolerate)
     // C1-C6: offset 1 (header preserved - user data is not disturbed)