[AZ-322] C10 DescriptorBatcher (faiss-cpu, OOM halve-retry)

Implements the C10 internal phase that walks every C6 tile, embeds
through C2's backbone via the AZ-321-produced engine, and rebuilds
the AZ-306 FAISS HNSW index in one atomic write.

- DescriptorBatcher with halve-and-retry OOM recovery (default 1 retry)
- BackboneEmbedder Protocol + C7EngineBackboneEmbedder default impl
- DescriptorBatchError for OOM / dim-mismatch / missing-output failures
- Empty-corpus surfaces as outcome=failure with explicit hint to run C11
- Per-10% progress callback + DEBUG logs (no engine bytes leaked)
- Consumer-side Protocol cuts (TilesByBboxBatchQuery, TilePixelOpener,
  DescriptorIndexRebuilder) so c10 stays within AZ-270 lint
- runtime_root.c10_factory adds build_descriptor_batcher + three
  C6->C10 adapters
- 16 unit tests covering AC-1..AC-10 + 2 NFRs + 4 supplemental
  (Protocol conformance, query pass-through, handle release, config)

Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/_autodev_state.md

@@ -6,11 +6,11 @@ step: 7
 name: Implement
 status: in_progress
 sub_step:
-  phase: 3
-  name: compute-next-batch
-  detail: "batch 35 complete (AZ-306 5pt; faiss-cpu PyPI strategy chosen over custom pybind11 wrapper); awaiting next batch selection"
+  phase: 4
+  name: batch-complete
+  detail: "batch 36 complete: AZ-322 implemented + tests + factory wiring; ready to chain to next batch"
 retry_count: 0
 cycle: 1
 tracker: jira
-last_completed_batch: 35
+last_completed_batch: 36
 last_cumulative_review: batches_31-33