[AZ-620] Phase B: build_pre_constructed seeds c6_descriptor_index + c6_tile_store

Second of six subtasks of AZ-618. Extends
airborne_bootstrap.build_pre_constructed(config) additively with the
two C6 storage entries on top of AZ-619's c13_fdr + clock contract:

- c6_descriptor_index: via storage_factory.build_descriptor_index
- c6_tile_store:       via storage_factory.build_tile_store

When BUILD_FAISS_INDEX=OFF, the lower-level RuntimeNotAvailableError
from the descriptor index factory is translated into an
AirborneBootstrapError that names the missing key
(c6_descriptor_index), the gating flag (BUILD_FAISS_INDEX), and the
consuming component slug(s) drawn from
AIRBORNE_REQUIRED_PRE_CONSTRUCTED_KEYS. The original error is
preserved as __cause__ so operators still see the upstream reason.

Tests: 3 new unit tests cover AC-620.1 + AC-620.2 (twice, with and
without a configured consumer, so the bootstrap fails loudly in
either branch). AZ-619 tests updated to add an autouse stub for the
Phase B builders (keeps them focused on Phase A keys) and to relax
the "exactly two keys" assertion to "AZ-619 keys remain present
under AZ-620 additivity" per the original test's own forward-pointer.

Bonus: ruff --fix removed 12 pre-existing UP037 quoted-annotation
warnings in airborne_bootstrap.py (covered by `from __future__ import
annotations`). All in modified-area scope per quality-gates.mdc.

Run: pytest tests/unit/runtime_root/ -q -> 15/15 passed in 1.06s.

Spec moved to _docs/02_tasks/done/ in the previous commit (audit-trail
backfill of batch_90 also landed there).

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

_docs/03_implementation/batch_91_cycle1_report.md

@@ -0,0 +1,67 @@
+# Batch 91 Report — AZ-620 Phase B: build_pre_constructed seeds c6_descriptor_index + c6_tile_store (cycle 1)
+
+**Batch**: 91
+**Date**: 2026-05-19
+**Context**: Product implementation (greenfield Step 7 — Implement; AZ-618 umbrella, Phase B of 6)
+**Tasks**: AZ-620 (3 cp) — 1 task
+**Cycle**: 1
+**Verdict**: COMPLETE — PASS (self-reviewed inline; 15/15 runtime_root tests green)
+
+## Summary
+
+Second of six subtasks decomposing the AZ-618 umbrella. Phase B extends `airborne_bootstrap.build_pre_constructed(config)` additively, adding the two C6 storage entries (`c6_descriptor_index`, `c6_tile_store`) on top of AZ-619's foundational `c13_fdr` + `clock`. Wraps the existing C6 factories so a `BUILD_FAISS_INDEX=OFF` mismatch surfaces a clear operator-facing `AirborneBootstrapError` instead of bubbling up the lower-level `RuntimeNotAvailableError` with no consuming-component context.
+
+### AZ-620 — build_pre_constructed Phase B (3 cp)
+
+* **`src/gps_denied_onboard/runtime_root/airborne_bootstrap.py`**:
+  * Imports `build_descriptor_index` + `build_tile_store` from `storage_factory`, and `RuntimeNotAvailableError` from `errors`.
+  * New public constant `FAISS_BUILD_FLAG = "BUILD_FAISS_INDEX"` (exported in `__all__`) — single source of truth for the flag name embedded in error messages.
+  * New module-private helpers:
+    * `_consumers_of_pre_constructed_key(key)` — reads `AIRBORNE_REQUIRED_PRE_CONSTRUCTED_KEYS` to find which slugs need a given key.
+    * `_configured_consumers_of_pre_constructed_key(config, key)` — narrows to consumers present in `config.components`; falls back to the full theoretical set when no components are configured (so error messages stay informative under bare `Config()`).
+    * `_build_c6_descriptor_index(config)` — calls `build_descriptor_index`; on `RuntimeNotAvailableError`, re-raises as `AirborneBootstrapError` naming `c6_descriptor_index`, `BUILD_FAISS_INDEX`, and the consuming component slug(s). Preserves the original error via `raise ... from exc`.
+    * `_build_c6_tile_store(config)` — thin pass-through to `build_tile_store` (no `BUILD_*` flag gates this side).
+  * `build_pre_constructed(config)` now returns four keys: `c13_fdr`, `clock`, `c6_descriptor_index`, `c6_tile_store`. Additivity preserved per AZ-620 Constraint "MUST be additive on top of AZ-619".
+  * Bonus hygiene (mechanical): `ruff --fix` removed 12 pre-existing UP037 quoted-annotation warnings in the same file (covered by `from __future__ import annotations` at line 49). All 12 lints were in the modified-area scope per `.cursor/rules/quality-gates.mdc`.
+
+* **`tests/unit/runtime_root/test_az619_pre_constructed_phase_a.py`**:
+  * New autouse fixture `_stub_c6_builders` monkeypatches `_build_c6_descriptor_index` + `_build_c6_tile_store` to opaque sentinels — keeps AZ-619 tests focused on Phase A keys without entangling them with the new Phase B integration paths (and avoids requiring real Postgres + FAISS for the AZ-619 cases).
+  * `test_phase_a_only_seeds_two_keys` renamed to `test_phase_a_keys_remain_present_under_az620_additivity`: now asserts AZ-619 keys are a SUBSET of the returned dict (not exact equality), matching the explicit forward-pointer that the original docstring carried ("this test will be relaxed at that point").
+
+* **`tests/unit/runtime_root/test_az620_pre_constructed_phase_b.py`** (new):
+  * `test_ac_620_1_adds_c6_descriptor_index_and_c6_tile_store` — stubs both C6 factories to identifiable mocks; asserts the bootstrap dict contains both Phase B keys referencing the patched returns, and AZ-619 keys are still present. Covers AC-620.1.
+  * `test_ac_620_2_build_flag_off_with_configured_c2_vpr_raises_named_error` — stubs `build_descriptor_index` to raise `RuntimeNotAvailableError` (mirroring real `BUILD_FAISS_INDEX=OFF` behaviour); builds a config with `c2_vpr.strategy="net_vlad"`; asserts `AirborneBootstrapError` is raised with all three names (`c6_descriptor_index`, `BUILD_FAISS_INDEX`, `c2_vpr`) and the original error preserved as `__cause__`. Covers AC-620.2.
+  * `test_ac_620_2_no_configured_consumer_still_raises_with_full_set` — defence-in-depth: even with no consumer configured, the bootstrap still fails loudly (rather than silently dropping the key); the message lists the full theoretical consumer set. Strengthens AC-620.2's operator-facing contract.
+
+## File Ownership
+
+* OWNED (component `runtime_root`):
+  * `src/gps_denied_onboard/runtime_root/airborne_bootstrap.py`
+  * `tests/unit/runtime_root/test_az619_pre_constructed_phase_a.py`
+  * `tests/unit/runtime_root/test_az620_pre_constructed_phase_b.py`
+* READ-ONLY: imports from `runtime_root.storage_factory` (same package), `runtime_root.errors`, `clock`, `fdr_client`, `config`, `c6_tile_cache.interface` (TYPE_CHECKING via storage_factory only).
+* No FORBIDDEN-zone writes.
+
+## Test Results
+
+* New unit tests: **3** under `tests/unit/runtime_root/test_az620_pre_constructed_phase_b.py` covering AC-620.1 + AC-620.2 (twice, defence-in-depth).
+* Updated unit tests: **5** AZ-619 tests still passing after autouse-stub fixture and relaxed-equality assertion.
+* AZ-591 regression (`test_az591_airborne_bootstrap.py`): **7/7 passing** — `AIRBORNE_REQUIRED_PRE_CONSTRUCTED_KEYS` table consistency + airborne registration semantics + tier isolation all unchanged.
+* Combined: `pytest tests/unit/runtime_root/ -q` → **15/15 passed in 1.06 s**.
+* No new ruff errors on touched files; 12 pre-existing UP037 warnings in the same file fixed.
+
+## Out of scope (deferred)
+
+* All other keys in `AIRBORNE_REQUIRED_PRE_CONSTRUCTED_KEYS` — AZ-621 (c7 inference), AZ-622 (c3 runtimes), AZ-623 (RANSAC + c5 helpers), AZ-624 (`main()` integration + AC-1..AC-5).
+* Real Postgres + FAISS integration testing — covered by the existing AZ-303 / AZ-305 / AZ-306 component-level tests; not duplicated at the bootstrap layer.
+* Mandatory-Tier-2 Jetson run — consolidated at AZ-624.
+
+## State
+
+* Spec moved: `_docs/02_tasks/todo/AZ-620_pre_constructed_phase_b_c6_storage.md` → `_docs/02_tasks/done/`.
+* Tracker: AZ-620 transitioned **To Do → In Progress** at batch start; will transition **In Progress → In Testing** after this commit (per implement skill Step 12).
+* `_docs/_autodev_state.md` advanced to `last_completed_batch: 91`.
+
+## Next Batch
+
+* **Batch 92**: AZ-621 (3 cp) — Phase C of AZ-618 umbrella. Adds `c7_inference` engine to `build_pre_constructed`. Depends on AZ-619 + AZ-620 (both will be in `done/` after this commit).