[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>

cpp/CMakeLists.txt

@@ -14,6 +14,8 @@ if(BUILD_GTSAM_BINDINGS)
   add_subdirectory(gtsam_bindings)
 endif()
 
-if(BUILD_FAISS_INDEX)
-  add_subdirectory(faiss_index)
-endif()
+# BUILD_FAISS_INDEX no longer adds a CMake target — AZ-306 chose the
+# `faiss-cpu` PyPI wheel over a custom pybind11 wrapper. The flag is
+# preserved in cmake/build_options.cmake because it's still consumed
+# at runtime by `gps_denied_onboard.runtime_root.build_flags` to gate
+# the FaissDescriptorIndex import in the storage factory.

cpp/faiss_index/CMakeLists.txt

@@ -1,4 +0,0 @@
-if(NOT BUILD_FAISS_INDEX)
-  return()
-endif()
-message(STATUS "[faiss_index] Placeholder; owned by C6 (AZ-306).")