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.
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.
PresenceEngine gains an overrides snapshot (child_id -> list of
PresenceOverride): is_present checks covering overrides BEFORE the
pattern and returns the override's flag as the answer — an absent
override on a normally-present day returns False, a present one on a
normally-absent day returns True, and an override on a child with no
schedule still applies (holidays work for the always-present sibling).
Snapshot validation mirrors the schedules': dict of lists keyed by the
override's own child id, exposed read-only. The DAO layer guarantees
no overlapping overrides per child; a hand-built snapshot that carries
them resolves deterministically to the LATEST start date (documented).
Tests: all three precedence cases, span edges (outside days stay on
the pattern), snapshot rejection matrix, deterministic overlap
tie-break. Suite: 995 passed.
Joshua King