Universal Spell Check v0.4.0

docs: align overlay docs with per-phase status and Done-before-restore

The loading overlay docs still described the old SetBusy(bool)
contract. Updated for the SetPhase pipeline shipped in 9f95eb1 and
1f125ea:

1. architecture.md — pipeline steps now show the three SetPhase
   milestones (Copying at run start, Sending before the request,
   Receiving after the response) and that Done fires in RunAsync
   before the clipboard restore; the overlay section documents the
   per-phase text, the sized-to-widest-string/no-wrap layout, and
   the OverlayHost STA thread (which the section had never caught
   up with).
2. watchlist.md — Loading overlay marshalling entry rewritten for
   OverlayHost BeginInvoke queueing, plus the phase-to-visibility
   contract and a warning that hiding after the clipboard restore
   re-introduces the stuck-overlay-after-failure bug.
3. conventions.md, NATIVE_APP_FUTURE_TODO.md — SetBusy references
   renamed; the stuck-overlay troubleshooting note now points at
   Done-before-restore.
4. src/CLAUDE.md — manual check 5 verifies all three phase texts
   and that the label is always a single line.
5. overview.md — file-map line mentions the per-phase status text.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>