Extract the panel snapshot builder into core (task 9047a97b)

The household snapshot assembly that lived inline in the coordinator's
_async_update_data is extracted into a pure, Home-Assistant-free
core/snapshot.py: the QuestInstanceView / ChildDaySnapshot /
NestQuestSnapshot dataclasses, the _cycle_day helper, and a new async
build_snapshot(database, settings, today, time_zone) builder.

The coordinator is now a thin DataUpdateCoordinator wrapper that
resolves the local timezone and settings and delegates to
build_snapshot; the dataclasses and _cycle_day are re-exported from the
coordinator module so existing sensor.py / binary_sensor.py imports keep
working unchanged (entity_ids and per-child instance attribute payload
shape unchanged). The API service's panel snapshot route (next task)
shares this one implementation.

Tests that monkeypatched derive_state on the coordinator module now
patch core.snapshot.derive_state (the module that now owns the call).
A new test seeds a household and asserts build_snapshot's output equals
the coordinator's snapshot for the same database and date. Suite green
at 1588.

tests/test_entity_matrix.py

@@ -186,18 +186,18 @@ async def test_entity_and_event_matrix(hass, make_entry) -> None:
     original_refresh = coordinator._async_update_data
 
     async def _future_dated_refresh():
-        import custom_components.nestquest.coordinator as coordinator_module
+        import custom_components.nestquest.core.snapshot as snapshot_module
 
-        real_derive = coordinator_module.derive_state
+        real_derive = snapshot_module.derive_state
 
         def _derive(instance, latest, today):
             return real_derive(instance, latest, today + datetime.timedelta(days=1))
 
-        coordinator_module.derive_state = _derive
+        snapshot_module.derive_state = _derive
         try:
             return await original_refresh()
         finally:
-            coordinator_module.derive_state = real_derive
+            snapshot_module.derive_state = real_derive
 
     coordinator._async_update_data = _future_dated_refresh
     await coordinator.async_refresh()