Tie coordinator wrapper to snapshot builder; single clock read

build_snapshot now takes now ONLY and derives today = now.date()
inside (P3-1): a single source of truth so the calendar date and
wall-clock time can never disagree across two reads. Coordinator
call site updated.

The expected completed_at in tests/test_core_snapshot.py is now built
from pinned_now.astimezone(datetime.timezone.utc).isoformat() instead
of a literal '+00:00' UTC string (P3-2), so the assertion is
timezone-independent rather than relying on the hass fixture's UTC
timezone.

Pin the coordinator's clock to pinned_now, run async_refresh, and
assert coordinator.data == built (the frozen dataclasses make == a
full deep compare) — the REQUIRED tie-in that proves the coordinator
wrapper actually delegates to build_snapshot, which the literal
assertions alone could not (P2).

custom_components/nestquest/coordinator.py

@@ -117,7 +117,6 @@ class NestQuestCoordinator(DataUpdateCoordinator):
 
     async def _async_update_data(self) -> NestQuestSnapshot:
         now = self._local_now()
-        today = now.date()
         return await build_snapshot(
-            self.database, self.settings, today, now
+            self.database, self.settings, now
         )