test : pin the chat perf invariants in the unit suite
Cover the fixes whose silent regression would be stale or wrong UI rather
than a crash: the turn-section cache must reuse unchanged turns yet
recompute on every field it compares; the sibling map must resolve the
same leaves after the leaf-walk memoization; the active conversation must
keep its identity through field updates; and the blob gates ( exec tail
window, plain-text result gate, search prefilter ) must keep accepting
what they gate. Only the risky invariants are pinned - no coverage for
coverage's sake.
Assisted-by: pi:zai-org/GLM-5.3
