Support Ctrl+Digit (1..9)

[jxdones]

Tapes that use Ctrl+1 through Ctrl+9 are not supported. xterm.js in legacy mode (used by ttyd) has no escape-sequence mapping for Ctrl+Digit combinations, so the keypress is silently dropped and never reaches the PTY.

Bubbletea uses the kitty keyboard protocol to handle these combinations. Without this, any app that binds ctrl+1-ctrl+9 (e.g. tab switching, workspace selection) cannot be demonstrated with VHS. I ran into this while recording a demo of my TUI app, It uses Ctrl+digit for navigation between tabs, and I couldn't capture those interactions at all. So I had to make changes locally.

Describe the solution you'd like
Emit the kitty keyboard protocol sequence (\x1b[<codepoint>;<modifier>u) directly via term.input() in ExecuteCtrl, the same approach already used by ExecuteScroll for term.scrollLines().

Also, a parser change to accept NUMBER tokens in parseCtrl(), following the same pattern as the arrow-key support added in #673.

Example in tape:
Ctrl+1
Ctrl+Shift+2
...

Describe alternatives you've considered

An alternative would be to attach a custom key event handler to xterm.js via attachCustomKeyEventHandler in the terminal setup. This intercepts the DOM keydown event and calls term.input() with the kitty sequence directly from JavaScript. However, this adds a global side effect at setup time, present for every tape even when Ctrl+digit is never used, and splits the execution logic across two files instead of keeping it self-contained in ExecuteCtrl.

Additional context

The solution I'm proposing follows the same pattern as the Ctrl+arrow key support added in #673: a one-line parser change to accept the new token type, and a new case in ExecuteCtrl for execution. The digit path reuses the same direct term.input() approach already used by ExecuteScroll (term.scrollLines()).

I have a local implementation of this working, including unit tests covering Ctrl+0 through Ctrl+9 and the Shift/Alt modifier combinations. I also have a GIF on my project's repo changing tabs using Ctrl+Digit. Happy to open a PR if the approach looks good to you.