Add BRK breakpoint dispatch to the break monitor

[mbrukner]

Summary

This builds on #37 by allowing emulation-mode BRK instructions to enter the same break-monitor flow as a Foenix/SysRq break.

• detect BRK from the stacked processor status in the IRQ handler
• dispatch BRK through the existing monitor save/resume path
• mark BRK-originated entries in the monitor header
• update JR2 SysRq detection for current $A2 machine IDs
• reserve banks $3b–$3d for monitor state and remove conflicting kernel-side screen preservation

Testing

• make jr.bin
• extensive BRK handling testing