Detect when a player shortcuts across geometry and skips checkpoint gates
by monitoring the spline projection delta. When the player jumps past
the configured skip threshold, they are teleported back to the last
legitimate checkpoint, shown a blinking "CHECKPOINT MISSED" banner,
and given a brief invulnerability window.
- Add checkpointMissSkip and checkpointMissMs config values
- Implement delta-based gate-skipping detection in kart logic
- Add HUD banner and sound feedback for missed checkpoints
- Make player sprite blink during the correction window
- Add verification tests for detection, correction, and clean-race false positives
- Make Tetris Attack portrait frame half-transparent