feat(IMP-08): U3 — frontend wire (zoneSections override)

Wires the frontend drag/drop zone assignment through to the backend
--override-section-assignment CLI flag.

PipelineOverrides gains an optional zoneSections field
(Record<string, string[]>) carrying canonical ordinal section ids
(e.g., "top": ["04-2-sub-1"]).

Vite middleware /api/run accepts overrides.zoneSections and forwards
each non-empty zone as `--override-section-assignment ZONE=sid[,sid]`.
Empty arrays and non-string sids are filtered to avoid bogus
assignments from a partially-built UI state.

Home.tsx builds the override with a diff-vs-default guard per Codex
Stage 3 R3 B3 fix : createInitialUserSelection seeds zone_sections with
the auto plan, so a literal copy would pollute backend assignment-source
provenance even on a fresh re-render. The diff compares each zone's
section list against sourcePlan.zones[].section_ids and only emits zones
that differ. Toast summary now reports zoneSections=N when forwarded.

Smoke verification : python -m src.phase_z2_pipeline samples/mdx_batch/04.mdx
test_imp08_smoke --override-section-assignment primary=04-2-sub-1 produces
section_assignment_plan with assignment_source=cli_override and
v4_selector_trace.candidates populated via the U1 alias resolver
(04-2-sub-1 -> 04-2.1 V4 entry).

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

Front/client/src/pages/Home.tsx

@@ -300,6 +300,35 @@ export default function Home() {
       if (zoneGeometries && Object.keys(zoneGeometries).length > 0) {
         overrides.zoneGeometries = zoneGeometries;
       }
+
+      // IMP-08 B-3 : zoneSections forward only when the user diverged from
+      // the auto plan. Codex Stage 3 R3 B3 fix : `createInitialUserSelection`
+      // seeds `zone_sections` with the default placement, so a literal copy
+      // would pollute backend assignment-source provenance even on a fresh
+      // re-render. Diff against `sourcePlan.zones[].section_ids` per zone and
+      // only emit zones whose section list differs.
+      const userZoneSections = state.userSelection.overrides.zone_sections;
+      if (userZoneSections) {
+        const defaultByZone = new Map<string, string[]>();
+        sourcePlan.zones.forEach((z) => {
+          defaultByZone.set(z.zone_id, z.section_ids);
+        });
+        const zoneSectionsDiff: Record<string, string[]> = {};
+        for (const [zoneId, sids] of Object.entries(userZoneSections)) {
+          if (!Array.isArray(sids)) continue;
+          const cleaned = sids.filter((s) => typeof s === "string" && s.trim());
+          const defaults = defaultByZone.get(zoneId) ?? [];
+          const sameAsDefault =
+            cleaned.length === defaults.length &&
+            cleaned.every((sid, i) => sid === defaults[i]);
+          if (!sameAsDefault) {
+            zoneSectionsDiff[zoneId] = cleaned;
+          }
+        }
+        if (Object.keys(zoneSectionsDiff).length > 0) {
+          overrides.zoneSections = zoneSectionsDiff;
+        }
+      }
     }
 
     setState((p) => ({ ...p, isLoading: true }));
@@ -310,6 +339,8 @@ export default function Home() {
         ? `(overrides: ${[
             overrides.layout && `layout=${overrides.layout}`,
             overrides.frames && `frames=${Object.keys(overrides.frames).length}`,
+            overrides.zoneSections &&
+              `zoneSections=${Object.keys(overrides.zoneSections).length}`,
           ]
             .filter(Boolean)
             .join(", ")})`

Front/client/src/services/designAgentApi.ts

@@ -251,6 +251,11 @@ export interface PipelineOverrides {
   /** zone_id (top/bottom/left/right/...) → slide-body 내부 0~1 비율.
    *  backend 의 build_layout_css 가 horizontal-2 / vertical-2 만 처리. */
   zoneGeometries?: Record<string, { x: number; y: number; w: number; h: number }>;
+  /** IMP-08 B-3 : zone_id -> list of section_id assignments
+   *  (canonical ordinal `${parent}-sub-${n}`). Only forwarded when the
+   *  user explicitly diverges from the auto plan; default placements
+   *  are not echoed back to avoid polluting override provenance. */
+  zoneSections?: Record<string, string[]>;
 }
 
 export async function runPipeline(