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/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(