Apply PR #144 review round 3: lifecycle tests for malformed-option fallback, comment/test hygiene

Add LIFECYCLE tests that wire an entry via make_entry(options=...) and
call async_setup_entry, covering the from_options_resilient fallback
production actually uses (P2): a fully malformed options dict
(horizon_days=True, day_rollover_time='99:99',
morning_summary_time='25:00') succeeds with the day-rollover listener
at the default 00:00 and the backfill over DEFAULT_HORIZON_DAYS; and a
bad sibling (horizon_days=5, morning_summary_time='25:00') keeps
horizon_days == 5. Swapping from_options_resilient for from_options in
setup now fails these tests.

Reword comments/tests describing review history rather than the code:
__init__.py setup comment, test_core_settings P2-3/P2-1/P2-5 markers,
the moved from_options horizon test, and the AST scanner docstrings.

Test hygiene: move freeze_nestquest_clock into tests/conftest.py and
use it from test_core_settings and test_timezones; simplify CORE_PKG to
a plain import plus Path(mod.__file__).parent; move
test_from_options_horizon_validates_and_defaults into test_core_settings.

1575 passed.

tests/conftest.py

@@ -22,6 +22,7 @@ pytest -q``); it is never meant to run against a real HA install.
 from __future__ import annotations
 
 import asyncio
+import datetime
 import importlib.util
 import inspect
 import sys
@@ -916,4 +917,40 @@ def make_config_flow_entry():
     def _factory(domain: str, entry_id: str = "existing"):
         return SimpleNamespace(domain=domain, entry_id=entry_id)
 
-    return _factory
+    return _factory
+
+
+def freeze_nestquest_clock(
+    monkeypatch, instant: datetime.datetime
+) -> tuple[datetime.datetime, list]:
+    """Freeze the integration's ``datetime`` clock to a fixed aware instant.
+
+    Only ``custom_components.nestquest``'s ``datetime`` binding is patched,
+    so the generation path's ``datetime.datetime.now`` reads this instant
+    (localized to whatever time zone it asks for) while every other module
+    and the test harness keep the real clock.  Returns ``(frozen, requested)``
+    where ``frozen`` is the instant normalized to UTC and ``requested`` is
+    the list of ``tzinfo`` objects the code under test passed to ``now`` —
+    so a caller can prove the generation path actually requested the
+    configured HA time zone rather than silently reading UTC.
+    """
+    import custom_components.nestquest as nestquest
+
+    frozen = instant.astimezone(datetime.timezone.utc)
+    requested: list = []
+
+    class _FrozenDatetime(datetime.datetime):
+        @classmethod
+        def now(cls, tz=None):
+            requested.append(tz)
+            if tz is None:
+                return frozen.replace(tzinfo=None)
+            return frozen.astimezone(tz)
+
+    fake_module = SimpleNamespace(
+        datetime=_FrozenDatetime,
+        timedelta=datetime.timedelta,
+        date=datetime.date,
+    )
+    monkeypatch.setattr(nestquest, "datetime", fake_module)
+    return frozen, requested