feat(#68): IMP-39 u1~u8 ranking_sort_policy single-source + backend↔frontend label-priority mirror

u1: templates/phase_z2/catalog/ranking_sort_policy.yaml — single-source policy
    (label_priority asc {use_as_is:0, light_edit:1, restructure:2, reject:3}
    + confidence desc + v4_rank asc tie-break).
u2: src/phase_z2_pipeline.py — apply_ranking_sort helper + lookup_v4_match_with_fallback
    applies policy AFTER IMP-38 raw-window selection (raw default_window + usable_count
    preserved on RAW all_judgments).
u3: src/phase_z2_pipeline.py — _build_application_plan_unit forwards ranking_sort_policy
    + sorted_candidate_evidence into Step 9 payload.
u4: Front/client/src/services/designAgentApi.ts — frame_candidates builder reads
    unit.sorted_candidate_evidence + unit.ranking_sort_policy first; local LABEL_PRIORITY
    retained only on warn-fallback path.
u5: tests/test_ranking_sort_policy.py — pure permutation coverage (sample-agnostic).
u6: tests/phase_z2/test_label_priority_synthetic.py + fixtures/ranking_sort_policy/
    synthetic_divergence.yaml — low-conf use_as_is behind high-conf restructure.
u7: tests/phase_z2/test_imp39_mdx04_env_toggle_e2e.py — samples/mdx_batch/04.mdx with
    AI_FALLBACK_ENABLED=off; backend selected_v4_rank == frontend frame_candidates[0].
u8: tests/phase_z2/test_imp39_corpus_audit.py — real corpus sweep over
    tests/matching/v4_full32_result.yaml (10 MDX sections); section IDs loaded
    dynamically (RULE 0 / RULE 7 sample-agnostic).

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

Front/client/src/services/designAgentApi.ts

@@ -565,6 +565,13 @@ export async function loadRun(runId: string): Promise<LoadRunResult> {
     // sort 우선순위 = label (use_as_is > light_edit > restructure > reject) + confidence desc.
     // 모두 reject 인 경우 confidence desc 만 적용 (사용자 명시).
     const TOP_N_FRAMES = 6;
+    // IMP-39 u4 (issue #68) — local LABEL_PRIORITY is now a documentation
+    // mirror of templates/phase_z2/catalog/ranking_sort_policy.yaml (u1).
+    // Primary ordering arrives pre-sorted from the backend selector
+    // (src/phase_z2_pipeline.py lookup_v4_match_with_fallback :1186-1196 +
+    // _build_application_plan_unit u3 payload fields). This constant is read
+    // ONLY on the warn-fallback path below (legacy fixtures pre-u3 / payload
+    // missing). Kept verbatim so the fallback ordering matches u1/u2 contract.
     const LABEL_PRIORITY: Record<string, number> = {
       use_as_is: 0,
       light_edit: 1,
@@ -576,9 +583,6 @@ export async function loadRun(runId: string): Promise<LoadRunResult> {
     //   2) unit.v4_all_judgments   (pre-IMP-05 audit array)
     //   3) unit.v4_candidates      (legacy minimal)
     // fallback_chain alias is intentionally NOT read (Stage 2 guardrail).
-    const candidateEvidence = Array.isArray(unit.candidate_evidence)
-      ? unit.candidate_evidence
-      : [];
     const candidateMap = new Map<string, any>();
     const pushCandidate = (c: any) => {
       if (!c) return;
@@ -586,15 +590,64 @@ export async function loadRun(runId: string): Promise<LoadRunResult> {
       if (!key) return;
       if (!candidateMap.has(key)) candidateMap.set(key, c);
     };
-    candidateEvidence.forEach(pushCandidate);
-    (unit.v4_all_judgments ?? []).forEach(pushCandidate);
-    (unit.v4_candidates ?? []).forEach(pushCandidate);
-    const rawSource = Array.from(candidateMap.values());
-    const v4Source = [...rawSource].sort((a: any, b: any) => {
-      const lp = (LABEL_PRIORITY[a.label] ?? 99) - (LABEL_PRIORITY[b.label] ?? 99);
-      if (lp !== 0) return lp;
-      return (b.confidence ?? 0) - (a.confidence ?? 0);
-    });
+
+    // IMP-39 u4 (issue #68) — primary path: consume the backend Step 9
+    // payload as the single source of ordering truth.
+    //   • ``unit.sorted_candidate_evidence`` = policy-sorted selector trace
+    //     (src/phase_z2_pipeline.py :4163, alias of selection_trace[
+    //     "candidates"] sorted by u2 at :1186-1196). Same IMP-05 L2 schema
+    //     consumed below (template_id, label, confidence, frame_number,
+    //     frame_id, rank, catalog_registered, capacity_fit, route_hint, ...).
+    //   • ``unit.ranking_sort_policy`` = full single-source policy dict
+    //     (policy_type / label_priority / unknown_label_priority /
+    //     tie_break_axes) forwarded for telemetry + fallback parity check.
+    // When both are present we feed sorted_candidate_evidence through the
+    // existing dedup map (first occurrence wins, mirrors backend
+    // ``seen_template_ids`` semantics at :1204-1236) and SKIP the local
+    // re-sort — backend "rank 1" then equals frontend frame_candidates[0]
+    // by construction (Stage 1 root-cause fix).
+    const sortedCandidateEvidence: any[] | null = Array.isArray(
+      unit.sorted_candidate_evidence,
+    )
+      ? unit.sorted_candidate_evidence
+      : null;
+    const rankingSortPolicy = unit.ranking_sort_policy ?? null;
+    const backendPolicyPayloadPresent =
+      sortedCandidateEvidence !== null &&
+      sortedCandidateEvidence.length > 0 &&
+      rankingSortPolicy !== null;
+
+    let v4Source: any[];
+    if (backendPolicyPayloadPresent) {
+      sortedCandidateEvidence!.forEach(pushCandidate);
+      v4Source = Array.from(candidateMap.values());
+    } else {
+      // IMP-39 u4 — warn-fallback path. Legacy fixtures predating u3 (or
+      // any code path that strips the payload) lack the backend-sorted
+      // evidence; ordering then derives from local LABEL_PRIORITY mirror.
+      // Warning surfaces drift in dev console without hard-failing the UI
+      // (graceful: production sample audit deck remains renderable).
+      if (typeof console !== "undefined" && typeof console.warn === "function") {
+        console.warn(
+          `[IMP-39 u4] unit ${unit.unit_id ?? "<unknown>"}: backend payload ` +
+            "missing ranking_sort_policy / sorted_candidate_evidence — " +
+            "falling back to local LABEL_PRIORITY (legacy fixture path).",
+        );
+      }
+      const candidateEvidence = Array.isArray(unit.candidate_evidence)
+        ? unit.candidate_evidence
+        : [];
+      candidateEvidence.forEach(pushCandidate);
+      (unit.v4_all_judgments ?? []).forEach(pushCandidate);
+      (unit.v4_candidates ?? []).forEach(pushCandidate);
+      const rawSource = Array.from(candidateMap.values());
+      v4Source = [...rawSource].sort((a: any, b: any) => {
+        const lp =
+          (LABEL_PRIORITY[a.label] ?? 99) - (LABEL_PRIORITY[b.label] ?? 99);
+        if (lp !== 0) return lp;
+        return (b.confidence ?? 0) - (a.confidence ?? 0);
+      });
+    }
     // ─── IMP-41 u4 — application_candidates enrichment (issue #70) ───────────
     // Backend Step 9 emits `unit.application_candidates[]` (src/phase_z2_pipeline.py
     // _application_candidates_for_unit, :3071-3092) one entry per v4 candidate with