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/client/src/services/designAgentApi.ts

@@ -345,13 +345,25 @@ export interface PipelineOverrides {
 
 export async function runPipeline(
   file: File,
-  overrides?: PipelineOverrides
+  overrides?: PipelineOverrides,
+  // IMP-43 (#72) u6 — optional prev RUN_ID for incremental rerun. When set,
+  // the vite plugin forwards `--reuse-from <PREV_RUN_ID>` to the backend
+  // and the pipeline resumes at Step 7 (Step 0/1/2/5/6 artifacts copied
+  // from the prior run). When omitted / empty, the POST body is
+  // byte-identical to pre-u6 (no reuseFromRunId key → no flag forwarded).
+  reuseFromRunId?: string,
 ): Promise<RunPipelineResult> {
   const content = await file.text();
+  const body: Record<string, unknown> = {
+    filename: file.name,
+    content,
+    overrides,
+  };
+  if (reuseFromRunId) body.reuseFromRunId = reuseFromRunId;
   const res = await fetch("/api/run", {
     method: "POST",
     headers: { "Content-Type": "application/json" },
-    body: JSON.stringify({ filename: file.name, content, overrides }),
+    body: JSON.stringify(body),
   });
   const data = (await res.json()) as RunPipelineResult;
   if (!res.ok && !data.run_id) {