Implement a way to interrupt mid-turn

Pressing your own timer in the middle of someone else's turn should:
1. Freeze the current player's timer
2. Start your timer
3. Upon releasing "priority" it returns to the current player.
4. This feature should, if possible, stack.

Functionality sketch:

No files or tests are named; start by locating the timer and priority-handling entry points. Verify the current player's timer freezes, the interrupter's timer starts, priority returns correctly on release, and nested interruptions stack when possible.

Written by the indexing model from the issue text.