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_header_box.h

@@ -41,13 +41,14 @@
 //
 // Returns:
 //   true  - C1-C6: Header preserved at offset 0, next at offset 1
-//   false - C0, C7: Header overwritten by next pointer at offset 0
+//   false - C0: Header overwritten by next pointer at offset 0
+//   Phase 13 v1: C7 returns false (default) or true (HAKMEM_TINY_C7_PRESERVE_HEADER=1)
 
 static inline bool tiny_class_preserves_header(int class_idx) {
 #if HAKMEM_TINY_HEADER_CLASSIDX
     // Delegate to tiny_layout_box.h specification (Single Source of Truth)
-    // next_off=0 → header overwritten (C0, C7)
-    // next_off=1 → header preserved (C1-C6)
+    // next_off=0 → header overwritten (C0, C7 default)
+    // next_off=1 → header preserved (C1-C6, C7 with HAKMEM_TINY_C7_PRESERVE_HEADER=1)
     return tiny_nextptr_offset(class_idx) != 0;
 #else
     // Headers disabled globally
@@ -87,7 +88,8 @@ static inline void tiny_header_write_if_preserved(void* base, int class_idx) {
 // ============================================================================
 //
 // Validates header ONLY if this class preserves headers.
-// For C0/C7, validation is impossible (next pointer is stored at offset 0).
+// For C0, validation is impossible (next pointer is stored at offset 0).
+// Phase 13 v1: C7 validation depends on HAKMEM_TINY_C7_PRESERVE_HEADER.
 //
 // Arguments:
 //   base      - BASE pointer (not user pointer)