v0.6.0 — burned-in captions everywhere

subtitles: 'burn' now works on every ffmpeg build. Instead of libass (which Homebrew's ffmpeg 8 no longer ships), each cue is rendered as a transparent caption pill by the same browser that records the tutorial, then composited with ffmpeg's built-in overlay filter — identical rendering on every machine, styled with CSS (captionStyle: { fontSizePx, maxWidthPx, bottomMarginPx }). Captions composite after scale and zoom, so --zoom never distorts them, and cue timing shares the same map as SRT/localization/idle-speedup. Closes #7.