[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>

tests/e2e/replay/_faiss_seed.py

@@ -0,0 +1,87 @@
+"""AZ-964 — seed a minimal empty HNSW32 + IndexIDMap2 FAISS index fixture.
+
+Shared by:
+
+* `scripts/mk_test_faiss_fixture.py` — invoked by the `tile-init`
+  setup service in `docker-compose.test.jetson.yml`.
+* `tests/e2e/replay/conftest.py::_build_operator_pre_flight_cache`
+  — the AZ-839 C3 fixture, which creates a fresh tmp `root_dir` per
+  test and needs an empty index there before `build_descriptor_index`
+  can call `FaissDescriptorIndex._load()` without raising
+  `IndexUnavailableError`.
+
+The seed produces three files under ``root_dir``:
+
+* ``descriptor.index``           — HNSW32 / IndexIDMap2 binary
+* ``descriptor.index.sha256``    — sha256 sidecar (verified by ``_load``)
+* ``descriptor.index.meta.json`` — metadata with matching
+  ``sidecar_sha256_hex`` (cross-checked by ``_load``)
+
+The default ``descriptor_dim=512`` + ``backbone_label="ultra_vpr"``
+mirror the prior in-script defaults; callers can override when seeding
+for a NetVLAD (4096) or DINOv2-VPR run (AZ-965 territory).
+"""
+
+from __future__ import annotations
+
+import hashlib
+import json
+from datetime import datetime, timezone
+from pathlib import Path
+
+import faiss  # type: ignore[import-untyped]
+
+__all__ = ["seed_empty_faiss_index"]
+
+_HNSW_M = 32
+_EF_CONSTRUCTION = 40
+_EF_SEARCH = 16
+
+
+def seed_empty_faiss_index(
+    root_dir: Path,
+    *,
+    descriptor_dim: int = 512,
+    backbone_label: str = "ultra_vpr",
+) -> Path:
+    """Create an empty valid HNSW32 FAISS index at ``root_dir/descriptor.index``.
+
+    Idempotent — re-running overwrites the prior fixture. Returns the
+    path to the written ``.index`` file.
+    """
+
+    root_dir.mkdir(parents=True, exist_ok=True)
+
+    inner = faiss.IndexHNSWFlat(descriptor_dim, _HNSW_M, faiss.METRIC_INNER_PRODUCT)
+    index = faiss.IndexIDMap2(inner)
+
+    idx_path = root_dir / "descriptor.index"
+    faiss.write_index(index, str(idx_path))
+    idx_bytes = idx_path.read_bytes()
+    sha256 = hashlib.sha256(idx_bytes).hexdigest()
+
+    (idx_path.parent / (idx_path.name + ".sha256")).write_text(
+        sha256, encoding="ascii"
+    )
+
+    meta = {
+        "descriptor_dim": descriptor_dim,
+        "n_vectors": 0,
+        "backbone_label": backbone_label,
+        "backbone_sha256_hex": "0" * 64,
+        "built_at": datetime.now(timezone.utc).isoformat(),
+        "hnsw_params": {
+            "m": _HNSW_M,
+            "ef_construction": _EF_CONSTRUCTION,
+            "ef_search": _EF_SEARCH,
+            "metric": "INNER_PRODUCT",
+        },
+        "sidecar_sha256_hex": sha256,
+        "file_path": str(idx_path),
+        "id_mapping": [],
+    }
+    (idx_path.parent / (idx_path.name + ".meta.json")).write_text(
+        json.dumps(meta, sort_keys=True, indent=2), encoding="utf-8"
+    )
+
+    return idx_path