fix: publish Twist instead of TwistStamped on /cmd_vel, improve keyboard overlay (#10)

- Change LCM message type from TwistStamped to Twist (matches DimOS convention)
- Channel: /cmd_vel#geometry_msgs.Twist (was geometry_msgs.TwistStamped)
- Twist encoding: 56 bytes (hash + 6 doubles), no Header overhead
- Fingerprint hash: 0x2e7c07d7cdf7e027 (verified against Python LCM reference)

Keyboard overlay improvements:
- Dark semi-transparent background panel (HUD style)
- Visual key grid showing Q/W/E/A/S/D with active state highlighting
- Green highlight on pressed keys, dim when idle
- STOP bar (space) with red flash on e-stop
- Shift speed indicator
- Always visible (dims when no keys active)
- Fix egui deprecation: Rounding -> CornerRadius, .rounding() -> .corner_radius()

All 27 tests pass.

Author: spomichter