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/sensor.py

@@ -39,6 +39,7 @@ from .coordinator import (
     NestQuestCoordinator,
     QuestInstanceView,
 )
+from .core.snapshot import instance_payload
 
 #: Home Assistant's state machine refuses states longer than 255
 #: characters (its hard recorder/state limit).  These sensors' states
@@ -63,40 +64,6 @@ def _state_within_limit(state: Any) -> Any:
     return state
 
 
-def _instance_payload(
-    instances: Iterable[QuestInstanceView], *, include_missed: bool
-) -> list[dict[str, Any]]:
-    """Shape instance dicts per ENTITIES-AND-SERVICES.md §1.
-
-    ``state`` maps the coordinator's derived ``done`` to the panel's
-    ``completed`` spelling; ``missed`` instances are omitted unless
-    ``include_missed`` (the admin payload, D-009, keeps them with
-    their ``missed`` state).
-    """
-    payload: list[dict[str, Any]] = []
-    for view in instances:
-        if view.state == "missed" and not include_missed:
-            continue
-        payload.append(
-            {
-                "id": view.instance_id,
-                "definition_id": view.definition_id,
-                "child_id": view.child_id,
-                "title": view.title,
-                "icon": view.icon,
-                "window": view.window,
-                "due_time": view.due_time,
-                "state": (
-                    "completed" if view.state == "done" else view.state
-                ),
-                "overdue": view.overdue,
-                "completed_at": view.completed_at,
-                "on_time": view.was_on_time,
-            }
-        )
-    return payload
-
-
 #: State reported by the next-quest sensor when the child owes nothing
 #: else today.  A documented sentinel, never an empty string (an empty
 #: state reads as "sensor went unavailable" in HA).
@@ -215,10 +182,10 @@ class NestQuestQuestsDueTodaySensor(_NestQuestChildDaySensor):
         if child is None:
             return None
         return {
-            "instances": _instance_payload(
+            "instances": instance_payload(
                 child.instances, include_missed=False
             ),
-            "admin_instances": _instance_payload(
+            "admin_instances": instance_payload(
                 child.instances, include_missed=True
             ),
             "child_id": child.child_id,