[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/_autodev_state.md

@@ -6,10 +6,10 @@ step: 7
 name: Implement
 status: in_progress
 sub_step:
-  phase: 12
-  name: archive
-  detail: "AZ-619 (Phase A) implemented + committed + tests green (12/12 incl. AZ-591 regression). Spec moved to done/. AZ-619 transitioned In Testing. Next batch = AZ-620 (Phase B: c6_descriptor_index + c6_tile_store, 3pt). Recommended session boundary here per 2026-05-18 lesson — fresh session picks up cleanly with AZ-620."
+  phase: 11
+  name: commit
+  detail: "batch 91 — AZ-620 committed; tracker → In Testing next"
 retry_count: 0
 cycle: 1
 tracker: jira
-last_completed_batch: 90
+last_completed_batch: 91