Address PR #142 review: core isolation invariant and shim fixes

- Point recurrence purity scan at custom_components.nestquest.core.recurrence
  (P2.1); the shim only re-exports, so the real model was no longer scanned.
- Make test_const/test_config_flow/test_dao_matrix module scans recursive
  (rglob) so custom_components/nestquest/core/ is covered (P2.2, P2.3);
  const.py name exclusion retained.
- Add test_core_isolation: AST-scan every core module for homeassistant
  imports, relative imports climbing out of core (level > 1), absolute
  imports reaching custom_components.nestquest.<not core>, and bare
  'import core'/'from core' (P2.4).
- Drop the private re-export of _month_end/_nth_weekday_of_month from the
  recurrence shim; those are test-only, so move the two test imports to
  custom_components.nestquest.core.recurrence (P2.1/P3.1).
- Fix shim docstrings and core/__init__.py to reference
  custom_components.nestquest.core.* instead of the unimportable 'core.*'
  paths (P3.2).
- Note the migrations logger namespace move to
  custom_components.nestquest.core.migrations in RELEASE_NOTES.md (P3.3).

RELEASE_NOTES.md

@@ -1,5 +1,19 @@
 # NestQuest Release Notes
 
+## Unreleased
+
+- **Storage layer extraction (task d0b691d8):** the SQLite storage layer
+  (schema, migrations, db, store, const, recurrence, DAOs) is bundled
+  under `custom_components/nestquest/core/`, with the integration's
+  original module names preserved as re-export shims.
+- **Logger namespace change (breaking for logging configuration):** the
+  migrations runner now logs under
+  `custom_components.nestquest.core.migrations` instead of
+  `custom_components.nestquest.migrations`. Users who tuned the old
+  logger name (e.g. in `logger.yaml` or `configuration.yaml`) must
+  update the entry to keep that level, as the old name no longer
+  receives migration log output.
+
 ## Version 0.5.0 — 2026-09-18
 
 ### Scope