chore: c6 docs-hygiene from cumulative_review_batches_28-30

Land F1+F2+F3 from the PASS_WITH_WARNINGS cumulative review of
batches 28-30 (AZ-305 / AZ-307 / AZ-308) before continuing to
batch 31. All three are bounded by the c6_tile_cache component;
no public API contract change beyond the new error re-export.

F1 (Medium / Architecture):
  Re-export CacheBudgetExhaustedError from c6_tile_cache package
  __init__ so consumers can catch the AZ-308 budget-exhaustion
  variant without widening to TileCacheError (which drops the
  needed_bytes / available_bytes / evicted_count diagnostics).

F2 (Medium / Architecture):
  Refresh the c6_tile_cache section of module-layout.md so the
  Public API line and the Internal-files list reflect what is
  actually on disk after batches 28-30 (drop the stale
  Tile / TileRecord / connection.py entries; add the AZ-305
  postgres_filesystem_store + tools.py, AZ-307 freshness_gate,
  AZ-308 cache_budget_enforcer entries; pivot the Public API
  bullet to the __init__.__all__ as canonical, mirroring the
  c7_inference section format).

F3 (Low / Maintainability):
  Promote the triplicate intra-module _iso_ts_now() helper into
  a single c6_tile_cache._timestamp.iso_ts_now and import it
  from postgres_filesystem_store, freshness_gate, and
  cache_budget_enforcer. FDR record envelope ts format now has
  one source of truth.

Test impact:
  tests/unit/c6_tile_cache: 105 passed, 57 skipped (pre-existing
  Docker-compose skip markers). No new tests required for F1/F2
  (re-export + doc) and F3 (pure refactor; existing tests assert
  FDR record shape, not the helper symbol).

Autodev state advanced to awaiting-invocation; next session
resumes greenfield Step 7 at batch 31 (AZ-298 TensorrtRuntime).

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

src/gps_denied_onboard/components/c6_tile_cache/__init__.py

@@ -39,6 +39,7 @@ from gps_denied_onboard.components.c6_tile_cache._types import (
 )
 from gps_denied_onboard.components.c6_tile_cache.config import C6TileCacheConfig
 from gps_denied_onboard.components.c6_tile_cache.errors import (
+    CacheBudgetExhaustedError,
     ContentHashMismatchError,
     FreshnessRejectionError,
     IndexBuildError,
@@ -60,6 +61,7 @@ register_component_block("c6_tile_cache", C6TileCacheConfig)
 __all__ = [
     "Bbox",
     "C6TileCacheConfig",
+    "CacheBudgetExhaustedError",
     "ContentHashMismatchError",
     "DescriptorIndex",
     "FreshnessLabel",

src/gps_denied_onboard/components/c6_tile_cache/_timestamp.py

@@ -0,0 +1,21 @@
+"""RFC 3339 UTC timestamp helper shared inside the c6_tile_cache component.
+
+Single source for the FDR record envelope ``ts`` field across
+``postgres_filesystem_store``, ``freshness_gate``, and
+``cache_budget_enforcer`` — formerly a triplicate ``_iso_ts_now`` per
+module (`cumulative_review_batches_28-30` F3). The format is wall-clock
+metadata about WHEN the FDR record was emitted; producers that need a
+Clock-driven payload field (e.g., ``age_seconds`` derived from an
+injected clock) MUST NOT route through this helper.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime, timezone
+
+__all__ = ["iso_ts_now"]
+
+
+def iso_ts_now() -> str:
+    """Return an RFC 3339 UTC timestamp with microsecond precision and a ``Z`` suffix."""
+    return datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%S.%fZ")

src/gps_denied_onboard/components/c6_tile_cache/cache_budget_enforcer.py

@@ -34,6 +34,7 @@ from dataclasses import dataclass
 from datetime import datetime, timezone
 from typing import TYPE_CHECKING, Final
 
+from gps_denied_onboard.components.c6_tile_cache._timestamp import iso_ts_now
 from gps_denied_onboard.components.c6_tile_cache._types import (
     TileId,
     TileMetadata,
@@ -80,16 +81,6 @@ class EvictionResult:
     freed_bytes: int
 
 
-def _iso_ts_now() -> str:
-    """RFC 3339 UTC timestamp with microsecond precision and ``Z`` suffix.
-
-    Used only on the FDR record envelope ``ts`` field — distinct from the
-    per-row ``accessed_at`` / ``evicted_at`` datetimes which use the same
-    wall-clock source but carry the operator-facing semantics.
-    """
-    return datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%S.%fZ")
-
-
 class CacheBudgetEnforcer:
     """LRU-driven 10 GiB hard-cap enforcer for the C6 tile cache.
 
@@ -300,7 +291,7 @@ class CacheBudgetEnforcer:
         self._fdr_client.enqueue(
             FdrRecord(
                 schema_version=CURRENT_SCHEMA_VERSION,
-                ts=_iso_ts_now(),
+                ts=iso_ts_now(),
                 producer_id=_PRODUCER_ID,
                 kind="c6.eviction_batch",
                 payload={

src/gps_denied_onboard/components/c6_tile_cache/freshness_gate.py

@@ -47,6 +47,7 @@ from typing import TYPE_CHECKING, Any, Final
 import psycopg
 from psycopg_pool import ConnectionPool
 
+from gps_denied_onboard.components.c6_tile_cache._timestamp import iso_ts_now
 from gps_denied_onboard.components.c6_tile_cache._types import (
     Bbox,
     FreshnessLabel,
@@ -116,16 +117,6 @@ class _Sector:
         return (self.bbox.max_lat - self.bbox.min_lat) * (self.bbox.max_lon - self.bbox.min_lon)
 
 
-def _iso_ts_now() -> str:
-    """RFC 3339 UTC timestamp with microsecond precision and ``Z`` suffix.
-
-    Used only on the FDR record envelope ``ts`` field, which is wall-clock
-    metadata about WHEN the record was emitted — distinct from the
-    Clock-driven ``age_seconds`` payload which uses the injected clock.
-    """
-    return datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%S.%fZ")
-
-
 class FreshnessGate:
     """Per-flight freshness gate (AZ-307).
 
@@ -374,7 +365,7 @@ class FreshnessGate:
         self._fdr_client.enqueue(
             FdrRecord(
                 schema_version=CURRENT_SCHEMA_VERSION,
-                ts=_iso_ts_now(),
+                ts=iso_ts_now(),
                 producer_id=_PRODUCER_ID,
                 kind="c6.freshness.rejected",
                 payload={
@@ -393,7 +384,7 @@ class FreshnessGate:
         self._fdr_client.enqueue(
             FdrRecord(
                 schema_version=CURRENT_SCHEMA_VERSION,
-                ts=_iso_ts_now(),
+                ts=iso_ts_now(),
                 producer_id=_PRODUCER_ID,
                 kind="c6.freshness.downgraded",
                 payload={

src/gps_denied_onboard/components/c6_tile_cache/postgres_filesystem_store.py

@@ -45,6 +45,7 @@ from psycopg.rows import dict_row
 from psycopg.types.json import Jsonb
 from psycopg_pool import ConnectionPool
 
+from gps_denied_onboard.components.c6_tile_cache._timestamp import iso_ts_now
 from gps_denied_onboard.components.c6_tile_cache._tile_pixel_handle import (
     TilePixelHandle,
 )
@@ -103,11 +104,6 @@ _ALLOWED_VOTING_TRANSITIONS = frozenset(
 )
 
 
-def _iso_ts_now() -> str:
-    """RFC 3339 UTC timestamp with microsecond precision and ``Z`` suffix."""
-    return datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%S.%fZ")
-
-
 class MmapTilePixelHandle(TilePixelHandle):
     """Read-only mmap view of a tile JPEG (Invariant I-4 read-only).
 
@@ -992,7 +988,7 @@ class PostgresFilesystemStore:
         )
         record = FdrRecord(
             schema_version=CURRENT_SCHEMA_VERSION,
-            ts=_iso_ts_now(),
+            ts=iso_ts_now(),
             producer_id=_PRODUCER_ID,
             kind="c6.write",
             payload={
@@ -1020,7 +1016,7 @@ class PostgresFilesystemStore:
             message = message[:_MAX_FDR_FAILURE_MSG_LEN] + "...<truncated>"
         record = FdrRecord(
             schema_version=CURRENT_SCHEMA_VERSION,
-            ts=_iso_ts_now(),
+            ts=iso_ts_now(),
             producer_id=_PRODUCER_ID,
             kind="c6.write_failed",
             payload={