Bundle core inside the integration

HACS and the manual install copy only custom_components/nestquest, so
the integration shipped with a top-level 'core' package that was never
deployed and failed with ModuleNotFoundError: No module named 'core'.

Move core/ into custom_components/nestquest/core/, repoint the
re-export shims to relative '.core.X' imports, and update tests
(import paths, monkeypatch targets, caplog logger) to the new
'custom_components.nestquest.core.X' location. The integration is now
self-contained: nothing under custom_components/nestquest reaches
outside its own tree, the migrations logger becomes
custom_components.nestquest.core.migrations, and the SQL-leak guard
allowlists point at the new core/ paths.

uv run pytest -q: 1496 passed.

tests/test_materialize.py

@@ -1039,7 +1039,7 @@ def test_regenerate_for_child_uses_single_anchor_across_midnight(
         clock["reads"] += 1
         return day0 if clock["reads"] == 1 else day1
 
-    import core.dao_instances as dao_instances_module
+    import custom_components.nestquest.core.dao_instances as dao_instances_module
 
     monkeypatch.setattr(dao_instances_module, "_today", _fake_today)