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 - 変更は必ず **箱 + 境界1箇所 + ENVで戻せる** で積む（Fail-fast、最小可視化）。
 - A/B は **同一バイナリでENVトグル**が原則（別バイナリ比較は layout が混ざる）。
+- SSOT運用（ころころ防止）: `docs/analysis/PHASE75_6_SSOT_POLICY_FAST_PGO_VS_STANDARD.md`
 - “削除して速い” は封印（link-out/大削除は layout tax で符号反転しやすい）→ **compile-out** を優先。
   - 診断: `scripts/box/layout_tax_forensics_box.sh` / `docs/analysis/PHASE67A_LAYOUT_TAX_FORENSICS_SSOT.md`
 
diff --git a/docs/analysis/PHASE75_6_SSOT_POLICY_FAST_PGO_VS_STANDARD.md b/docs/analysis/PHASE75_6_SSOT_POLICY_FAST_PGO_VS_STANDARD.md
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+# Phase 75-6: SSOT Policy — FAST PGO vs Standard (stop “ころころ” drift)
+
+## Problem statement
+
+After Phase 75, we observed:
+- Phase 75 win is **real** (C5/C6 inline slots improve D vs A in both Standard and FAST PGO).
+- Absolute “baseline” numbers **move** across commits/builds (especially with PGO), causing SSOT confusion (“ころころ変わる”).
+
+This document defines a stable SSOT policy that keeps Box Theory iteration reliable.
+
+## Definitions
+
+### Standard binary
+- `./bench_random_mixed_hakmem`
+- Used for: correctness, production-like behavior, “stable across code refactors”
+
+### FAST PGO binary
+- `./bench_random_mixed_hakmem_minimal_pgo`
+- Used for: competitive speed tracking vs mimalloc (best-case tuned build)
+- Caveat: more sensitive to build/layout drift than Standard
+
+### SSOT harness
+- `scripts/run_mixed_10_cleanenv.sh`
+- Must pin the binary explicitly via `BENCH_BIN=...` when comparing Standard vs FAST.
+
+## SSOT policy (two-track)
+
+### Track A (Decision SSOT): same-binary A/B
+
+For accepting a feature (GO/NEUTRAL/NO-GO), the primary truth is:
+- **same binary**, **ENV toggle only**
+- Example: Phase 75 4-point matrix within the same binary.
+
+This avoids layout tax from “different binaries” and is aligned with prior learnings:
+- link-out / large pruning can flip signs due to layout.
+
+### Track B (Competitive SSOT): FAST PGO ratio vs mimalloc
+
+For “how close to mimalloc”, use FAST PGO:
+- `BENCH_BIN=./bench_random_mixed_hakmem_minimal_pgo`
+- mimalloc is still a separate binary reference (layout differs), so treat ratio as “headline”, not proof of a micro-change.
+
+## Practical rules to prevent SSOT drift
+
+1. **Never mix Standard numbers into FAST ratio tables**
+   - Standard A/B results are valid, but not directly comparable to FAST baseline.
+
+2. **When reporting a result, always include:**
+   - binary (`bench_random_mixed_hakmem` vs `bench_random_mixed_hakmem_minimal_pgo`)
+   - workload (`ITERS`, `WS`, `RUNS`)
+   - key ENV knobs (`WARM_POOL_SIZE`, `C5/C6 inline`, etc.)
+
+3. **If FAST PGO baseline changes across commits**
+   - treat it as “baseline rebase event”, not automatically “regression”
+   - confirm using `scripts/box/layout_tax_forensics_box.sh` + perf stat deltas (IPC/branch/cache)
+
+4. **Do not demote FAST PGO SSOT solely from one episode**
+   - use Track A (same-binary A/B) to validate the optimization first
+   - then decide whether FAST PGO is “worth maintaining” based on ongoing ROI
+
+## Recommended next action after Phase 75-5
+
+- Keep Phase 75 (C5/C6) promoted for Standard and for FAST builds.
+- Treat Phase 69’s 62.63M as historical reference, not guaranteed to reproduce on later commits.
+- Proceed with Phase 76 using Track A for GO decisions, and Track B for periodic headline updates.
+