Implements the remaining committed Sprint 1 stories (Batches 3-5):
- CLI emits line-oriented PROGRESS events (startup/render/complete/failure)
  on stdout, throttled to >=1/sec, alongside the existing stderr/exit-code
  contract.
- New EnvelopeRenderer.Desktop WinForms app lets an operator pick template/
  CSV/output paths and launch a render without blocking the UI.
- Desktop app streams and parses the CLI's progress events live, shows
  active status, and displays a completion summary with elapsed time and
  totals.

Verified end-to-end against the real 392-row sample CSV (real success and
real forced-failure runs); 106/106 tests passing. Sprint 1's goal is met
and its backlog is fully Done; state.md and backlog/sprints/sprint-1.md
are updated to hand off to Sprint Review.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>