fix(tui): preserve live parts during session hydration

[kitlangton]

Summary

• preserve live message and part updates that arrive while initial TUI session history hydration is still in flight
• merge individual live parts over the stale response instead of replacing them with older HTTP snapshot data
• add a regression test reproducing live assistant text being replaced with an empty hydrated part

Root Cause

SyncProvider.session.sync() fetched session history asynchronously and then replaced message/part state wholesale. If a live message.part.updated event arrived before that response completed, the older response could overwrite visible assistant text with stale content until a later reload.

This is an independently reproduced TUI data race; it does not claim to fully explain or fix the separately observed OpenTUI blank-region report.

Verification

• bun run test -- test/cli/cmd/tui/sync-live-hydration.test.tsx test/cli/cmd/tui/sync.test.tsx test/cli/cmd/tui/sync-undefined-messages.test.tsx
• bun typecheck
• bunx oxlint packages/opencode/src/cli/cmd/tui/context/sync.tsx packages/opencode/test/cli/cmd/tui/sync-live-hydration.test.tsx (existing warnings in sync.tsx, no lint errors)

[kitlangton]

/review

[github-actions]

Suggestion: consider only calling touchPart after this delta is actually applied. Right now a delta for a part that is not in local state still marks the part as live, then the hydration merge treats the HTTP snapshot as stale and drops it because there is no current part to preserve. Moving the revision touch below the !parts / !result.found guards would keep discarded deltas from deleting the hydrated part.