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>

scripts/run_mixed_10_cleanenv.sh

@@ -11,6 +11,9 @@ runs=${RUNS:-10}
 
 # Force known research knobs OFF to avoid accidental carry-over.
 export HAKMEM_TINY_HEADER_WRITE_ONCE=${HAKMEM_TINY_HEADER_WRITE_ONCE:-0}
+export HAKMEM_TINY_C7_PRESERVE_HEADER=${HAKMEM_TINY_C7_PRESERVE_HEADER:-0}
+export HAKMEM_TINY_TCACHE=${HAKMEM_TINY_TCACHE:-0}
+export HAKMEM_TINY_TCACHE_CAP=${HAKMEM_TINY_TCACHE_CAP:-64}
 export HAKMEM_MALLOC_TINY_DIRECT=${HAKMEM_MALLOC_TINY_DIRECT:-0}
 export HAKMEM_ENV_SNAPSHOT_SHAPE=${HAKMEM_ENV_SNAPSHOT_SHAPE:-0}
 export HAKMEM_FREE_TINY_FAST_MONO_DUALHOT=${HAKMEM_FREE_TINY_FAST_MONO_DUALHOT:-0}
@@ -20,4 +23,3 @@ for i in $(seq 1 "${runs}"); do
   echo "=== Run ${i}/${runs} ==="
   HAKMEM_PROFILE="${profile}" ./bench_random_mixed_hakmem "${iters}" "${ws}" 1 2>&1 | rg "Throughput" || true
 done
-