[AZ-964] FAISS index bootstrap for AZ-839 fixture + build flag

AZ-964 SHIPPED — AZ-840 orchestrator test moves past FAISS gate.

Changes:
* tests/e2e/replay/_faiss_seed.py — extracts the empty HNSW32
  seeding logic from scripts/mk_test_faiss_fixture.py into a
  reusable test-infra module: seed_empty_faiss_index(root_dir,
  *, descriptor_dim=512, backbone_label="ultra_vpr") -> Path.
* scripts/mk_test_faiss_fixture.py rewritten as a thin CLI shim
  importing the same helper. compose `tile-init` contract is
  preserved.
* tests/e2e/replay/conftest.py::_build_operator_pre_flight_cache
  now calls seed_empty_faiss_index(cache_root) immediately before
  build_descriptor_index(config), so the factory's _load() finds
  a valid .index + .sha256 + .meta.json triplet at the fixture's
  override root_dir. populate_c6_from_route later in the fixture
  rebuilds the real index once route tiles are downloaded.
* docker-compose.test.jetson.yml: BUILD_PYTORCH_FP16_RUNTIME: "ON"
  added to e2e-runner.environment. Scope creep documented honestly
  in the spec — Tier-2 surfaced this third config gap on the same
  fixture chain while validating AZ-964 (RuntimeNotAvailableError:
  ... the flag is OFF). One-line wiring; the dustynv/l4t-pytorch
  base image bakes the Tegra-tuned PyTorch wheel and
  pytorch_fp16_runtime.py exists, so flag flip is sufficient.

Tier-2 verdict (4F / 48P / 3S / 1XF / 1XP in 86.07s, 0 errors —
was 2 errors before this commit): AZ-840 orchestrator test moves
from ERROR at FAISS gate to SKIP at empty-backbones gate — exactly
the AZ-965 gate AZ-964 AC-3 promised. test_operator_pre_flight_
integration SKIPs cleanly too. The 4 derkachi_1min ESKF-divergence
FAILs are constant across all three runs today (AZ-963 path,
independent of orchestrator chain).

Three Tier-2 runs today on the orchestrator chain:
  i.   pre-AZ-962: SKIP at env-var gate
  ii.  post-AZ-962: ERROR at FAISS gate
  iii. post-AZ-964: SKIP at backbones gate (AZ-965)

Cycle-4 e2e gate still NOT GREEN. Orchestrator chain remaining =
AZ-965 (NetVLAD backbone provisioning); 60s smoke chain remaining
= AZ-963 (ESKF divergence). OKVIS2 deferral directive unchanged.

Pre-existing yamllint false positive on docker-compose.test.jetson
.yml:185 (sibling `volumes:` keys flagged as duplicates without
respecting parent-key scope) — PyYAML parses cleanly with no
duplicates and docker-compose accepts the file at runtime.

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

docker-compose.test.jetson.yml

@@ -169,6 +169,13 @@ services:
       # `replay_runner` fixture trips that gate without this line.
       BUILD_CSV_REPLAY_ADAPTER: "ON"
       BUILD_FAISS_INDEX: "ON"
+      # AZ-964: build_inference_runtime gates pytorch_fp16 behind
+      # this flag. The dustynv/l4t-pytorch base image bakes the
+      # Tegra-tuned PyTorch wheel, so the strategy module imports
+      # cleanly when the flag is ON. build_engine_compiler (called
+      # by the AZ-839 fixture) requires c7 inference runtime, so
+      # the flag must be ON for the orchestrator test to run.
+      BUILD_PYTORCH_FP16_RUNTIME: "ON"
       # AZ-962: the AZ-839 C3 fixture (operator_pre_flight_setup) skips
       # the AZ-840 orchestrator test when this var is missing. The YAML
       # bind-mounted at /opt/configs/operator_replay.yaml declares the