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_recurrence_model.py

@@ -276,7 +276,7 @@ def test_recurrence_module_imports_only_stdlib() -> None:
     from pathlib import Path
 
     source = Path(
-        __import__("custom_components.nestquest.recurrence",
+        __import__("custom_components.nestquest.core.recurrence",
                    fromlist=["__file__"]).__file__
     ).read_text()
     offenders = _scan_forbidden_imports(source)