feat: keystroke overlay

[dwmkerr]

Summary

Add keystroke overlay/subtitles to recordings showing special keys as they're pressed, similar to popular terminal recording tools.

Use Case

When recording terminal sessions (especially vim, tmux, or other keyboard-heavy tools), viewers often can't see what keys are being pressed. An overlay showing keystrokes helps viewers:

• Learn keyboard shortcuts
• Follow along with complex key sequences
• Understand what's happening in the recording

Proposal

Add keystroke overlay support for recordings:

shell_record_start({
  session_id: "...",
  keystroke_overlay: true,
  overlay_position: "bottom"  // or "top"
})

The overlay would show:

• Modifier keys: Ctrl+C, Ctrl+Z, Alt+Tab
• Special keys: Esc, Enter, Tab, ↑, ↓, ←, →
• Vim-style notation: <C-w>, <Esc>, <CR>

Examples

Tools that do this well:

• Screenkey (Linux)
• KeyCastr (macOS)
• asciinema with key overlay plugins

Considerations

• Configurable display duration for each keystroke
• Option to show only modifier/special keys vs all keystrokes
• Styling options (font size, position, background opacity)