Extract the storage layer into core

core/__init__.py

@@ -0,0 +1,11 @@
+"""NestQuest storage layer.
+
+A self-contained, Home-Assistant-free package holding the NestQuest
+SQLite storage layer: schema DDL, the versioned migration runner, the
+async connection wrapper, the path resolver, the domain constants and
+recurrence model, and the typed DAOs.  Nothing in this package imports
+``homeassistant``; the integration wires it into HA at its own layer
+(constructing :class:`core.db.NestQuestDatabase` with an executor
+adapter backed by ``hass.async_add_executor_job``).
+"""
+from __future__ import annotations