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_sensors.py

@@ -238,16 +238,16 @@ async def test_due_today_payload_omits_missed_for_panel_includes_for_admin(
     entry, coordinator, (ada, bo, cory) = await _setup_seeded_entry(
         hass, make_entry
     )
-    import custom_components.nestquest.coordinator as coordinator_module
+    import custom_components.nestquest.core.snapshot as snapshot_module
 
     tomorrow = _today() + datetime.timedelta(days=1)
-    real_derive_state = coordinator_module.derive_state
+    real_derive_state = snapshot_module.derive_state
 
     def _derive_against_pinned_future(instance, latest, today):
         return real_derive_state(instance, latest, tomorrow)
 
     monkeypatch.setattr(
-        coordinator_module, "derive_state", _derive_against_pinned_future
+        snapshot_module, "derive_state", _derive_against_pinned_future
     )
     await coordinator.async_refresh()