## Summary
- Wraps store-proxied array iteration in `untrack()` in two
`createEffect` blocks and one `createMemo` in `message-section.tsx` to
prevent SolidJS from creating O(n) per-element reactive subscriptions on
every run
- Replaces `ids.includes()` with `Set.has()` for O(1) cleanup lookups in
the part-count tracking effect
## Problem
Two `createEffect` blocks in `message-section.tsx` iterate the
`messageIds()` store proxy array inside a tracked reactive context. This
causes SolidJS to create **O(n) per-element subscriptions** on every
run. When any element changes, all n subscriptions fire, re-running the
entire effect — resulting in **O(n²) total work**.
Additionally, the cleanup loop in the part-count tracking effect uses
`ids.includes(trackedId)` which is O(n) per tracked ID, compounding to
O(n²).
For long-running sessions with large message history (e.g. 7569
messages), this caused **~4.8 seconds of input latency** when sending a
new prompt.
## Fix
1. **Timeline sync effect (~line 738):** Wrap entire body in
`untrack()`, replace `ids.slice()` with `[...ids]` to snapshot without
proxy tracking
2. **Part-count tracking effect (~line 891):** Wrap iteration in
`untrack()`, replace `ids.includes()` with `new Set(ids).has()` for O(1)
lookups
3. **`lastAssistantIndex` memo:** Read message records via `untrack()`
to avoid O(n) subscriptions on part-level updates
## Result
On a 7569-message session: prompt input latency reduced from **~4.8s to
~42ms** (114x improvement).
Reconcile optimistic user messages by replacing the oldest synthetic pending message when the server-backed message arrives. Stop sending prompt part ids and rely on message-level replacement so v1.2.25 validation passes without duplicating optimistic content.
