feat: add asciicast v2 output format

[veeceey]

Adds support for generating .cast (asciicast v2) files as an output format, so you can do:

Output demo.cast

This has been a requested feature since the early days (#109) - being able to export recordings as asciicast means you can use them with asciinema players, convert to SVG with svg-term-cli, or generate GIFs with agg.

How it works

• Added Asciicast field to VideoOutputs so .cast is a first-class output alongside GIF/MP4/WebM
• During recording, terminal buffer snapshots are captured each frame (only when .cast output is requested to avoid overhead)
• At render time, GenerateAsciicast writes the asciicast v2 JSON Lines format: a header line with version/width/height/timestamp, followed by "o" (output) events with deduplicated content

Testing

$ go test ./...
ok      github.com/charmbracelet/vhs    4.806s
ok      github.com/charmbracelet/vhs/lexer      5.044s
ok      github.com/charmbracelet/vhs/parser      5.272s
ok      github.com/charmbracelet/vhs/token      5.482s

Closes #109

[veeceey]

Test Results

$ go build ./...
# no errors

$ go test ./...
ok      github.com/charmbracelet/vhs    4.806s
ok      github.com/charmbracelet/vhs/lexer      5.044s
ok      github.com/charmbracelet/vhs/parser      5.272s
ok      github.com/charmbracelet/vhs/token      5.482s

All existing tests pass with the new code.

[moritzdietz]

Great PR and indeed something a log of people were longing for!

No snark, but may I ask why you chose to implement v2 instead of v3 of the asciicast file format?
See https://blog.asciinema.org/post/three-point-o/