Audit follow-up. ChatBar subscribed to the whole `$statusItemsBySession` (a
computed that rebuilds the entire map) + `$previewStatusBySession` maps just to
derive a boolean, so every per-item status mutation (a subagent tick, the 5s
background poll) and every OTHER session's change re-rendered the ~1.4k
component. The queue hook likewise subscribed to the whole `$queuedPromptsBySession`
map.
- Add `useSessionStatusPresence` — a coarse edge (useSyncExternalStore) that
flips only when the stack shows/hides; ChatBar uses it for the styling
data-attr instead of the two map subscriptions.
- Add generic `useSessionSlice(store, key)` — subscribes to one session's array,
bailing out when other sessions churn (the plain atom keeps per-key refs
stable). The queue hook now reads its slice through it.
Result: ChatBar re-renders only when the stack's presence flips or this session's
queue changes — not on background/subagent status streaming or other sessions.
Verified: typecheck clean, 0 lint errors, composer tests 39/40 (pre-existing
attachments failure unrelated).
