Phase 13 v1 + E5-2 retest: Both NEUTRAL, freeze as research boxes

Phase 13 v1: Header Write Elimination (C7 preserve header)
- Verdict: NEUTRAL (+0.78%)
- Implementation: HAKMEM_TINY_C7_PRESERVE_HEADER ENV gate (default OFF)
- Makes C7 nextptr offset conditional (0→1 when enabled)
- 4-point matrix A/B test results:
  * Case A (baseline): 51.49M ops/s
  * Case B (WRITE_ONCE=1): 52.07M ops/s (+1.13%)
  * Case C (C7_PRESERVE=1): 51.36M ops/s (-0.26%)
  * Case D (both): 51.89M ops/s (+0.78% NEUTRAL)
- Action: Freeze as research box (default OFF, manual opt-in)

Phase 5 E5-2: Header Write-Once retest (promotion test)
- Verdict: NEUTRAL (+0.54%)
- Motivation: Phase 13 Case B showed +1.13%, re-tested with dedicated 20-run
- Results (20-run):
  * Case A (baseline): 51.10M ops/s
  * Case B (WRITE_ONCE=1): 51.37M ops/s (+0.54%)
- Previous test: +0.45% (consistent with NEUTRAL)
- Action: Keep as research box (default OFF, manual opt-in)

Key findings:
- Header write tax optimization shows consistent NEUTRAL results
- Neither Phase 13 v1 nor E5-2 reaches GO threshold (+1.0%)
- Both implemented as reversible ENV gates for future research

Files changed:
- New: core/box/tiny_c7_preserve_header_env_box.{c,h}
- Modified: core/box/tiny_layout_box.h (C7 offset conditional)
- Modified: core/tiny_nextptr.h, core/box/tiny_header_box.h (comments)
- Modified: core/bench_profile.h (refresh sync)
- Modified: Makefile (add new .o files)
- Modified: scripts/run_mixed_10_cleanenv.sh (add C7_PRESERVE ENV)
- Docs: PHASE13_*, PHASE5_E5_2_HEADER_WRITE_ONCE_* (design/results)

Next: Phase 14 (Pointer-chase reduction, tcache-style intrusive LIFO)

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

core/box/tiny_layout_box.h

@@ -79,14 +79,29 @@ static inline size_t tiny_user_offset(int class_idx) {
 // Offset for storing the freelist next pointer inside a freed block.
 // This is distinct from tiny_user_offset():
 //   - User offset is always +1 in header mode.
-//   - Next offset is 0 for C0/C7 (cannot preserve header while free), else 1.
+//   - Next offset:
+//     - C0: always 0 (16B, cannot fit header+next)
+//     - C1-C6: always 1 (header-preserving)
+//     - C7: 0 (default) or 1 (Phase 13 v1: header-preserving)
 static inline size_t tiny_nextptr_offset(int class_idx) {
 #if HAKMEM_TINY_HEADERLESS
     (void)class_idx;
     return 0;
 #elif HAKMEM_TINY_HEADER_CLASSIDX
-    // Bit pattern: C0=0, C1-C6=1, C7=0 → 0b01111110 = 0x7E
-    return (0x7Eu >> ((unsigned)class_idx & 7u)) & 1u;
+    // Phase 13 v1: C7 preserve header gate
+    // Bit pattern (default): C0=0, C1-C6=1, C7=0 → 0b01111110 = 0x7E
+    // Bit pattern (C7 preserve): C0=0, C1-C7=1 → 0b11111110 = 0xFE
+    unsigned int base_pattern = 0x7Eu;  // default: C7 offset=0
+
+    // Phase 13 v1: Gate for C7 header-preserving layout
+    if (class_idx == 7) {
+        extern int tiny_c7_preserve_header_enabled(void);
+        if (tiny_c7_preserve_header_enabled()) {
+            base_pattern = 0xFEu;  // C7 offset=1 (header-preserving)
+        }
+    }
+
+    return (base_pattern >> ((unsigned)class_idx & 7u)) & 1u;
 #else
     (void)class_idx;
     return 0u;