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>

Front/vite.config.ts

@@ -543,6 +543,13 @@ function vitePluginPhaseZApi(): Plugin {
               // (e.g., "top": ["03-1-sub-1"]). Forwarded as --override-section-assignment.
               zoneSections?: Record<string, string[]>;
             };
+            // IMP-43 (#72) u6 — optional PREV_RUN_ID to reuse Step 0/1/2/5/6
+            // artifacts from a prior run and resume execution at Step 7.
+            // Lives at the payload root (NOT under `overrides`) because the
+            // backend u1 post-merge guard rejects most override axes when
+            // --reuse-from is supplied. Absent / empty = full pipeline
+            // (byte-identical to pre-u6 spawn).
+            reuseFromRunId?: string;
           };
           try {
             payload = JSON.parse(body);
@@ -554,7 +561,7 @@ function vitePluginPhaseZApi(): Plugin {
             return;
           }
 
-          const { filename, content, overrides } = payload;
+          const { filename, content, overrides, reuseFromRunId } = payload;
           if (!filename || typeof content !== "string") {
             res.writeHead(400, { "Content-Type": "application/json" });
             res.end(
@@ -638,6 +645,19 @@ function vitePluginPhaseZApi(): Plugin {
               );
             }
           }
+          // IMP-43 (#72) u6 — --reuse-from <PREV_RUN_ID> forward. Backend
+          // (u1) parses this flag, validates the snapshot, copies Step
+          // 0/1/2/5/6 artifacts from data/runs/<PREV_RUN_ID>/phase_z2 into
+          // the new run_dir, and resumes execution at Step 7. The post-merge
+          // guard at the same site rejects --override-layout /
+          // --override-zone-geometry / --override-section-assignment /
+          // --override-image with axis-named fail-closed exit; only
+          // --override-frame (above) is preserved. Truthy check excludes
+          // empty string + undefined so an invalid argument never reaches
+          // argparse.
+          if (reuseFromRunId && typeof reuseFromRunId === "string") {
+            cliArgs.push("--reuse-from", reuseFromRunId);
+          }
           console.log(
             `[phase-z-api] spawn pipeline: run_id=${runId}, mdx=${mdxPath}, args=${JSON.stringify(cliArgs.slice(2))}`
           );