Throttle ACP plan update snapshots

[nikita-ashihmin]

Summary

• coalesce streamed Markdown plan snapshots to one update every 150 ms
• synchronously flush and deduplicate final snapshots at item, turn, permission, and disposal boundaries
• serialize plan sends so timer flushes cannot overtake completed snapshots

Tests

• stress coverage for 200 small plan deltas
• completed/interrupted turn flush and timer cleanup
• duplicate completed snapshot suppression
• independent plan item IDs and in-flight timer/completion ordering
• final plan snapshot precedes plan-review permission
• npm run typecheck
• npm test (340 passed, 28 skipped)
• npm run build