Subtitle command for text overlays on recordings

[gtmax]

Proposal

Add a Subtitle command that renders text overlays on terminal recordings. This is useful for demos, tutorials, and walkthroughs where you want to explain what's happening on screen.

Related: #173 (Progress bars & Overlays)
Related: #164 (display key names — the overlay infrastructure could be extended for keystroke display)
Related: #735 (Allow specifying subtitles in tape files)

Proposed syntax

Subtitle "This is what's happening"
Sleep 2s
Subtitle "Now something else"
Sleep 2s
Subtitle ""  # hide

With configurable settings:

Set SubtitleFontSize 28
Set SubtitleColor "#ffffff"
Set SubtitleBackground "rgba(0,0,0,0.75)"
Set SubtitlePosition "bottom"   # top, center, bottom
Set SubtitlePadding 12
Set SubtitleBorderRadius 8

The DSL is deliberately kept simple — covers common styling needs, and the implementation can be extended later without changing the tape syntax.

Example

Here's a real-world demo using subtitles to narrate a terminal workflow (wotr):

Implementation

I've put together a draft PR (#726) with a canvas-based overlay approach:

• A separate <canvas> element is drawn with the subtitle text on each frame
• Captured as a third PNG stream alongside text and cursor
• Composited in ffmpeg after all terminal styling (padding, margins, window bar)
• Zero overhead when no Subtitle commands are used

I considered two alternatives:

HTML/CSS overlay — would give richer styling, but Element.Screenshot() in CDP doesn't preserve transparency. Only Canvas.toDataURL() returns alpha-intact PNGs, which VHS needs for its ffmpeg pipeline.

ASS subtitle burn-in — richer text styling (outlines, shadows, fade animations), but requires ffmpeg compiled with libass. Standard Homebrew ffmpeg doesn't include it, which would break the install experience.

The canvas approach requires no additional dependencies and works with any ffmpeg build.

Would love feedback on the approach and syntax!

[tlipinski]

I've just tried it and it did excellent job!
https://github.com/tlipinski/rura/blob/7271091f5da104cb8a5d13fb846abff2725abd3e/demo.gif
I tried doing it manually with subtitles editor and gif to mkv conversion but... just no:/
With this feature there were no issues, intuitive usage, huge time saver.
Thanks!