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).

tests/test_config_flow.py

@@ -140,7 +140,7 @@ def test_flow_strings_match_translations(
 def test_no_hardcoded_domain_or_db_filename_in_new_modules() -> None:
     """Ensure no module other than const.py hardcodes the domain or db filename."""
     pkg_dir = Path(config_flow.__file__).parent
-    py_files = [f for f in pkg_dir.glob("*.py") if f.name not in ("const.py",)]
+    py_files = [f for f in pkg_dir.rglob("*.py") if f.name not in ("const.py",)]
 
     assert len(py_files) > 0, "No python modules found in package to check"