[AZ-341] C2 FAISS HNSW retrieve wiring (FaissBridge + AZ-507 cut)

Shared retrieve_topk plumbing for every concrete C2 VprStrategy:
- FaissBridge centralises the c6 search_topk → VprResult pipeline,
  the defended-in-depth INV-4 check (exactly k, distance-ascending),
  the WARN-threshold check on distances[0], optional per-frame DEBUG
  log, and one `vpr.retrieve_topk` FDR record per call with latency
  measurement.
- DescriptorIndexCut Protocol — consumer-side structural cut of c6
  DescriptorIndex.search_topk (AZ-507); keeps c2_vpr c6-import-free.
- C2VprConfig gains warn_top1_threshold + debug_per_frame_distances
  knobs with validators.
- KNOWN_PAYLOAD_KEYS registers vpr.retrieve_topk for the FDR record
  schema with payload {frame_id, backbone_label, top10_distances,
  latency_us}; companion fixture added to the AZ-272 roundtrip suite.
- 22 unit tests cover AC-1..AC-11 + NFR-perf microbench (p95 ≤ 0.5 ms)
  + constructor and retrieve-argument validation.

Verdict: PASS_WITH_WARNINGS (2 Low findings — duplicated ISO-ts
helper across c2/c5/c11/c12, captured in AZ-508 hygiene PBI;
spec-listed but unused `normaliser` parameter dropped — INV-3 makes
the embedding L2-normalised at the strategy's `embed_query`).

Tests: 1565 passed / 80 skipped (was 1543; +22 new tests).
Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/_autodev_state.md

@@ -4,11 +4,11 @@
 flow: greenfield
 step: 7
 name: Implement
-status: not_started
+status: in_progress
 sub_step:
-  phase: 0
-  name: awaiting-invocation
-  detail: ""
+  phase: 7
+  name: batch-loop
+  detail: "batch 45 — AZ-341 (C2 FAISS retrieve wiring)"
 retry_count: 0
 cycle: 1
 tracker: jira