commitSnapshot set the full fetched history then trimmed — briefly handing the
whole transcript to <For>. Since Yoga (WASM) layout memory is grow-only, even a
transient over-cap mount permanently ratchets the high-water mark, partly
defeating the cap when resuming a large session (a real one has ~1980 messages).
Slice to MESSAGE_CAP BEFORE the first setState so resume mounts at most the cap.
c40d3172ac