perf: polish context scan performance

[mnismt]

Summary

• Coalesce and abort stale select-all scans, use cursor traversal, and surface scan progress
• Add token-count request IDs and reuse size stats from counting results
• Use indexed tree state for child merges and folder lookups

Checks

• pnpm check-types
• pnpm lint
• pnpm -C src/webview-ui test --run
• git diff --check