Test fix: build the frozen-snapshot engine on the caller's list

Review round 3 (Codex gpt-5.6-terra, LOW): the helper copied the list,
so the mutation could never have affected even a shallow snapshot.
The engine is now constructed directly on the caller's list, making
the freeze regression-proof. Suite green before commit: 997 passed.

tests/test_presence.py

@@ -577,11 +577,18 @@ def test_engine_overlapping_snapshot_uses_latest_start() -> None:
 
 
 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."""
+    """The overrides snapshot must be truly read-only: the engine is
+    built directly on the CALLER's list, so mutating that list after
+    construction would change answers under a shallow copy — under the
+    frozen snapshot it must not."""
     entry = PresenceOverride(1, "2026-01-06", "2026-01-06", False)
     entries = [entry]
-    engine = _engine_with_overrides(*entries)
+    schedule = PresenceSchedule(
+        1, 2, ANCHOR, {0: {0, 1, 2, 3, 4, 5, 6}, 1: set()}
+    )
+    engine = PresenceEngine(
+        {1: schedule}, overrides={1: entries}  # caller-owned list
+    )
     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...