Speed up OR search, fd filename search, and preview

[oahsiao]

Summary

Performance improvements to the search backend (P0 + P1 from the optimization review).

P0 — search core

• OR content search → single ripgrep process. Multi-keyword OR previously spawned one ripgrep per keyword and traversed the filesystem N times. It now emits multiple -e patterns to a single process, scanning the tree once. rgSearchArgs accepts a keyword array; using -e also protects patterns starting with -.
  • Behavior note: a line matching several keywords now counts as 1 (matching line) instead of summing per keyword — arguably more correct.

P1 — filename search + preview

• fd filename search runs keywords concurrently (Promise.all) to overlap filesystem I/O instead of one scan per keyword.
• Preview reads files asynchronously (fs.promises.readFile) so the main process is no longer blocked on large files.
• Preview lowercases each line once (not once per keyword) and collects match line numbers as a pre-sorted, de-duplicated array, removing the Set + sort.

Testing

• npm test — all 5 self-tests pass.
• End-to-end smoke tests with real rg/fd against the repo:
  • OR content search (single process): 30 files
  • AND intersection: 19 files
  • fd OR (parallel): rg.js, fd.js
  • async preview: builds correctly