[AZ-306] C6 FaissDescriptorIndex (faiss-cpu, HNSW32)

Production-default DescriptorIndex strategy backed by the faiss-cpu
PyPI wheel (>=1.7,<2.0). Implements the AZ-303 Protocol surface end
to end: HNSW32 + IndexIDMap2 search, atomic three-file rebuild
(.index + .sha256 sidecar + .meta.json), triple-consistency load
check, mmap-backed reads with IO_FLAG_MMAP|IO_FLAG_READ_ONLY, optional
warm-up query at construction, FAISS RuntimeError rewrap to
IndexUnavailableError / IndexBuildError, and FaissDescriptorIndex.from_config
classmethod wired into runtime_root.storage_factory.

The original spec required a custom pybind11 wrapper over a vendored
FAISS HEAD; the user opted for the upstream faiss-cpu wheel after
research fact #92 confirmed ARM64 wheel availability for Jetson and
the existing pyproject.toml already pinned faiss-cpu. cpp/faiss_index/
placeholder removed; BUILD_FAISS_INDEX flag retained as a
runtime/factory gate (no native target). Spec rewritten end-to-end and
archived to _docs/02_tasks/done/.

C6TileCacheConfig extended with faiss_index_path and
faiss_warmup_query_path fields. tests/conftest.py sets
KMP_DUPLICATE_LIB_OK=TRUE to remediate the macOS faiss/torch libomp
duplicate-load abort during pytest (no-op on CI Linux). 21 new tests
cover AC-1..12 + 2 NFRs + from_config smoke; AZ-303 protocol-conformance
fake updated with from_config classmethod.

Tests: 124/124 c6_tile_cache pass; 1334 project-wide pass; 1
pre-existing OKVIS2 submodule failure unrelated.

Doc sync: module-layout.md, components/08_c6_tile_cache/description.md
§5, batch_35_cycle1_report.md.

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

pyproject.toml

@@ -63,6 +63,17 @@ dependencies = [
     # standard "two majors of compatibility" pattern other deps in
     # this file use.
     "cryptography>=43.0,<46.0",
+    # AZ-306 / E-C6: production-default `DescriptorIndex` strategy.
+    # `FaissDescriptorIndex` builds `IndexHNSWFlat` + `IndexIDMap2`,
+    # serialises via `faiss.write_index`, and mmap-loads via
+    # `faiss.read_index(..., IO_FLAG_MMAP | IO_FLAG_READ_ONLY)`.
+    # Promoted from the legacy `[indexing]` extras to main deps because
+    # the production airborne binary ALWAYS needs the descriptor index
+    # (no "Tier-0 dev runtime can ship without faiss" path remains).
+    # Major-version bound (<2.0) follows the same pattern as other
+    # third-party deps in this file. Research fact #92 + arch tech-stack
+    # both pin upstream FAISS via this PyPI distribution.
+    "faiss-cpu>=1.7,<2.0",
 ]
 
 [project.optional-dependencies]
@@ -89,9 +100,6 @@ telemetry = [
     "jetson-stats>=4.2",
     "pynvml>=11.5",
 ]
-indexing = [
-    "faiss-cpu>=1.7",
-]
 
 [project.scripts]
 gps-denied-replay = "gps_denied_onboard.cli.replay:main"