feat(#72): IMP-43 u1~u8 --reuse-from incremental rerun (Step 0/1/2/5/6 reuse + Step 7+ re-execute)

u1 argparse --reuse-from PREV_RUN_ID + post-merge fail-closed guard (rejects
layout/zone_geometry/zone_section/image override axes by name; only
--override-frame is preserved).
u2 src/phase_z2_reuse_snapshot.py — JSON-only Step 6 snapshot with mdx_sha256
integrity key and {value, source_path, upstream_step} provenance per axis
(pickle forbidden per Stage 2 guardrail).
u3 _write_reuse_snapshot at the Step 6 boundary; soft-fails to stderr without
aborting the seed run.
u4 prev_run_dir RO copy of step00/01/02/05/06 + _reuse_snapshot.json into
new run_dir, state rehydration, reuse marker, frame-override application on
restored units, Step 7+ resume.
u4b fail-closed for missing prev_run_dir / missing/corrupt/invalid snapshot /
mdx_sha256 mismatch / accidental new==prev write, with value+path+upstream
diagnostics per axis.
u5 reuse_from Optional[str] threaded through run_phase_z2_mvp1 signature and
CLI dispatch; default None preserves byte-identical pre-IMP-43 behavior.
u6 Front /api/run optional reuseFromRunId forwarding (vite.config.ts +
designAgentApi.ts + run_pipeline_reuse_from.test.ts).
u7a fast CI equivalence (1 mdx × 1 layout × 2 frames); step13 whitelist =
run_id/timestamps/prev_run_id only. u7b 3 layouts × 3 mdx × 32 frames
sweep gated by pytest.mark.sweep (registered in pyproject.toml; default CI
must use -m 'not sweep').
u8 scripts/measure_reuse_savings.py argv-driven A/B/C harness with frame
pin self-discovery + seed-time exclusion; status board §8 TBD anchor
(issue-body 50-70% / 10-20s→3-8s claim explicitly unverified, not mirrored).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/architecture/PHASE-Z-PIPELINE-STATUS-BOARD.md

@@ -182,6 +182,27 @@ Step 0 (사전 준비) 의 Figma → HTML 변환은 *precondition phase 의 작
 
 ---
 
+## 8. IMP-43 (#72) `--reuse-from` measured savings
+
+> Stage 2 §u8 binding contract: the issue-body 50–70% / 10–20s → 3–8s claim is **unverified** and is **not** mirrored here. Numbers below come from `scripts/measure_reuse_savings.py` on the project reference host; until that script is run and the values committed, every cell stays `TBD`.
+
+| axis | value |
+|---|---|
+| measurement script | `scripts/measure_reuse_savings.py` |
+| reuse boundary (Stage 1 lock) | Step 0 / 1 / 2 / 5 / 6 only; Step 7+ re-executes |
+| full rerun seconds (p50) | TBD |
+| full rerun seconds (p95) | TBD |
+| reuse seconds (p50) | TBD |
+| reuse seconds (p95) | TBD |
+| reuse / full ratio (p50) | TBD |
+| last measured | TBD (date / host / mdx / iterations) |
+
+Run protocol (per iteration): `(A)` seed → `(B)` full rerun with one self-discovered `--override-frame` pin → `(C)` `--reuse-from <seed>` with the same pin. The `(A)` seed time is reported separately and **not** included in the B-vs-C comparison — the reuse path's whole point is that the seed already exists from a prior interactive run.
+
+Invocation: `python -m scripts.measure_reuse_savings samples/mdx_batch/02.mdx --iterations 5` (mdx is argv-driven; the script does not pin a sample internally).
+
+---
+
 ## 사용 방법
 
 - 새 작업 들어오면 → 본 board 의 *어느 step* 의 status 를 바꾸는 작업인지 식별