Review fix: freeze the engine's overrides snapshot

Review round 1 (Codex gpt-5.6-terra, MEDIUM): the overrides mapping
was shallow-copied, leaving caller-owned lists mutable through
_overrides — a later append would change is_present results. Each
validated list is copied into an immutable tuple before wrapping;
tests prove post-construction appends change nothing and the snapshot
refuses mutation. Suite: 996 passed.

custom_components/nestquest/presence.py

@@ -384,10 +384,17 @@ class PresenceEngine:
                     )
         # Read-only snapshot: reassignment is refused and no mutator
         # exists, so a built engine answers consistently for its
-        # lifetime.
+        # lifetime.  The overrides lists are copied to immutable
+        # tuples — a shallow mapping copy alone would leave caller-
+        # owned lists reachable through _overrides, and mutating one
+        # later would change is_present results.
         object.__setattr__(self, "_schedules", MappingProxyType(dict(schedules)))
         object.__setattr__(
-            self, "_overrides", MappingProxyType(dict(overrides))
+            self,
+            "_overrides",
+            MappingProxyType(
+                {child_id: tuple(entries) for child_id, entries in overrides.items()}
+            ),
         )
 
     def is_present(self, child_id: object, date: object) -> bool:

tests/test_presence.py

@@ -574,3 +574,19 @@ def test_engine_overlapping_snapshot_uses_latest_start() -> None:
     assert engine.is_present(1, datetime.date(2026, 1, 9)) is True
     # Jan 11-12: only the later override covers -> present.
     assert engine.is_present(1, datetime.date(2026, 1, 11)) is True
+
+
+def test_engine_override_lists_are_frozen() -> None:
+    """The overrides snapshot must be truly read-only: mutating the
+    caller's list after construction must not change answers."""
+    entry = PresenceOverride(1, "2026-01-06", "2026-01-06", False)
+    entries = [entry]
+    engine = _engine_with_overrides(*entries)
+    assert engine.is_present(1, datetime.date(2026, 1, 6)) is False
+    entries.append(PresenceOverride(1, "2026-01-07", "2026-01-07", False))
+    # The late append must NOT change the engine's answers...
+    assert engine.is_present(1, datetime.date(2026, 1, 7)) is True
+    with pytest.raises(TypeError):
+        engine._overrides[1].append(entry)
+    with pytest.raises(AttributeError, match="immutable snapshot"):
+        engine._overrides = {}