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/store.py

@@ -1,7 +1,9 @@
-"""Re-export of :mod:`core.store` for the integration package.
+"""Re-export of :mod:`custom_components.nestquest.core.store` for the integration package.
 
-The path resolver lives in :mod:`core.store` (extracted in task
-d0b691d8) and is Home-Assistant-free: :func:`core.store.async_get_db_path`
+The path resolver lives in
+:mod:`custom_components.nestquest.core.store` (extracted in task
+d0b691d8) and is Home-Assistant-free:
+:func:`custom_components.nestquest.core.store.async_get_db_path`
 takes the full database path.  This module re-exports it and provides
 the original ``async_get_db_path(hass)`` signature so the integration
 resolves the path from ``hass.config.path`` exactly as before.
@@ -23,7 +25,8 @@ async def async_get_db_path(hass: HomeAssistant) -> Path:
     """Return the absolute path to the NestQuest SQLite database file.
 
     Resolves the database file from ``hass.config.path`` and delegates to
-    :func:`core.store.async_get_db_path`, which ensures the parent
-    directory exists off the event loop via :func:`asyncio.to_thread`.
+    :func:`custom_components.nestquest.core.store.async_get_db_path`,
+    which ensures the parent directory exists off the event loop via
+    :func:`asyncio.to_thread`.
     """
     return await _core_async_get_db_path(hass.config.path(SQLITE_DB_FILENAME))