feat(recording): add KeyCastr-style keypress overlays

[opencode-agent]

Summary

• add an opt-in KeyCastr-style overlay for press, enter, and arrow operations
• preserve keypress timing through recording export, clips, speed changes, holds, and restarts
• support Effect drivers, scripts, and start --record --keypress-overlay / send workflows
• keep batched ui.type text out of the overlay because it does not represent physical keystroke timing

Behavior

• shows up to three recent keypress pills for 1.2 seconds
• includes the overlay in both Drive-rendered screenshots and MP4 recording frames
• extends a final static recording long enough to show and clear its last keypress
• constrains long labels and small viewports without changing capture dimensions

Checks

• bun run check
• bun run --bun vitest run --exclude test/driver/index.test.ts (226 passed)
• focused recording/UI suite (43 passed)
• detached restart CLI integration test
• completed-timeline recording CLI integration test
• visual validation against the checked-in Tokyo Night theme-picker capture (1180×680; intended diff limited to the overlay)

The complete Effect suite reached 236 passing tests; one existing recording lifecycle test could not open the host's libx264 encoder and failed before writing frames.

Requested by: @nexxeln (nexxel via Slack)