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).

custom_components/nestquest/db.py

@@ -1,6 +1,6 @@
-"""Re-export of :mod:`core.db` for the integration package.
+"""Re-export of :mod:`custom_components.nestquest.core.db` for the integration package.
 
-The async SQLite wrapper lives in :mod:`core.db` (extracted in task
+The async SQLite wrapper lives in :mod:`custom_components.nestquest.core.db` (extracted in task
 d0b691d8) and is Home-Assistant-free: :class:`NestQuestDatabase` takes
 an executor callable rather than a ``hass`` object.  This module
 re-exports it and provides :func:`make_database`, a thin adapter that