v0.4.0 — failure diagnostics

Real apps fail in interesting ways; now the pipeline tells you how. Every step failure throws a StepError with artifact paths: a screenshot at failure and the recent browser console/pageerror/failed-request log. Run with --debug for the full treatment: a Playwright trace (npx playwright show-trace trace.zip), the complete console log, and before/after screenshots per step — work dir always kept. doctor now also flags ffmpeg builds without libass (subtitles: 'burn' needs it; Homebrew's ffmpeg 8 dropped it), and burn mode fails fast with a clear message instead of a cryptic filter error. Closes #6.