feat: typewriter text entry

[dwmkerr]

Summary

Add optional keystroke delay to shell_send for typewriter-style text entry during recordings.

Use Case

When creating demos or recordings, text appearing character-by-character looks more natural and helps viewers follow along.

Proposal

Add optional keystroke_delay_ms parameter to shell_send:

shell_send({
  session_id: "...",
  input: "kubectl get pods\r",
  keystroke_delay_ms: 100  // sends each character with 100ms delay
})

• If omitted or 0: current behavior (entire string sent at once)
• If set: characters sent one at a time with specified delay between each

Implementation notes

For smooth recordings, keystroke_delay_ms should roughly match the frame interval:

• 10 FPS → 100ms delay = ~1 char/frame
• 30 FPS → 33ms+ delay works well

No changes needed to GIF rendering - the existing fixed-FPS capture naturally picks up the intermediate states.

[dwmkerr]

Implementation consideration: FPS interaction

The keystroke delay should interact well with recording FPS for smooth results:

• At 10 FPS (100ms/frame), a 100ms keystroke delay = ~1 char/frame ✓
• At 30 FPS (33ms/frame), a 100ms keystroke delay = ~3 frames/char ✓

Recommendation: keystroke_delay_ms should be >= 1000 / fps for best visual effect.

Alternative approach: Instead of purely time-based frame capture, consider capturing a frame on each keystroke event during typewriter mode. This guarantees each character gets its own frame regardless of timing alignment.

[dwmkerr]

Dependency Update

This feature depends on #41 (recording evaluation workflow) for visual verification of keystroke capture quality.

Also related to #42 (adaptive frame timing) - both features benefit from variable per-frame delays to accurately capture keystrokes without frame loss.