Move panel payload shaping into core; pin the snapshot clock

PR #148 review (CHANGES_REQUIRED) follow-up:

- Move the panel payload dict shaper (_instance_payload) from the
  HA-importing sensor.py into core/snapshot.py as instance_payload, and
  have sensor.py import it from core. The API panel route now shares ONE
  shaping implementation with the HA sensor layer (done -> completed,
  on_time passthrough, missed omitted unless include_missed).
- build_snapshot takes the HA-local now explicitly instead of
  reading datetime.now(time_zone) internally, so date and time come from
  a single clock read and the overdue check is pinned in tests. The
  coordinator passes the same now it already resolved; time_zone is
  dropped (now is timezone-aware).
- Strengthen tests/test_core_snapshot.py with explicit literal expected
  values instead of self-equality: an AWAY child (2-week absent-week
  schedule + a presence override => next_present), a due-timed quest
  exercised as overdue under a pinned 12:00 now, a completed quest
  (done -> completed, on_time True), cycle_day == 2, and a constructed
  missed view proving omission from the panel payload and retention in
  the admin payload.
- Docstring cleanups: state plainly that settings is currently
  unused (no 'wired' implication); correct the coordinator docstring
  (settings are passed in, not resolved off the entry); trim the
  coordinator __all__ to what other modules actually import.
- Derive the seed/materialize date from coordinator._local_now() so the
  seed date and snapshot date cannot diverge near midnight; drop the
  redundant bo/ghost/list_children leftovers from the prior seed.

custom_components/nestquest/coordinator.py

@@ -14,10 +14,11 @@ layers.
 The snapshot assembly itself lives in the Home-Assistant-free
 :mod:`custom_components.nestquest.core.snapshot` module
 (:func:`~core.snapshot.build_snapshot`); this coordinator is now a thin
-HA wrapper that resolves the local timezone and settings off the config
-entry and delegates to that builder.  The API service's panel snapshot
-route shares the same builder, so both consumers render ONE snapshot
-shape.
+HA wrapper that resolves the local timezone and reads the settings handed
+into its constructor, then delegates to that builder, passing the SAME
+HA-local ``now`` it resolved in one clock read.  The API service's panel
+snapshot route shares the same builder, so both consumers render ONE
+snapshot shape.
 
 Presence is resolved through the SAME engine the materializer uses
 (:class:`~.presence.PresenceEngine`): schedules and overrides are read
@@ -53,7 +54,6 @@ from .core.snapshot import (
     ChildDaySnapshot,
     NestQuestSnapshot,
     QuestInstanceView,
-    _cycle_day,
     build_snapshot,
 )
 
@@ -62,8 +62,6 @@ __all__ = [
     "NestQuestCoordinator",
     "NestQuestSnapshot",
     "QuestInstanceView",
-    "_cycle_day",
-    "build_snapshot",
 ]
 
 
@@ -120,7 +118,6 @@ class NestQuestCoordinator(DataUpdateCoordinator):
     async def _async_update_data(self) -> NestQuestSnapshot:
         now = self._local_now()
         today = now.date()
-        time_zone = ZoneInfo(self.hass.config.time_zone)
         return await build_snapshot(
-            self.database, self.settings, today, time_zone
+            self.database, self.settings, today, now
         )