NMI break-to-monitor support (freeze / inspect / resume)

[mbrukner]

Adds a hardware-NMI (RESTORE key + Foenix modifier) break-to-monitor dispatch, letting a debugger freeze any running program, inspect it, and resume it exactly. Two files change: f256/jr.asm (the hw_nmi dispatch, replacing the old debug stub) and kernel/kernel.asm (a one-line-guarded timebase freeze in tick).

What it does

hw_nmi:

• Gates on the Foenix modifier; has a nested-break guard; declines a break into the monitor itself (when the victim's slot 5 is the monitor's reserved RAM bank).
• Freezes the victim: saves registers/SP/MMU/io, and saves+clears the text/color screen into reserved RAM banks.
• Copies the two-bank monitor image from flash into reserved RAM banks, maps both into the victim's LUT slots 4+5, and JSRs the fixed break entry under the victim's map (so the monitor sees the victim's address space).
• On resume: restores the victim's slots, screen, io/MMU/SP/registers and RTIs back exactly.

Resume transparency

Two subtleties are handled so resume is invisible to the victim:

• Kernel timers: tick checks platform.nmi_in_progress and skips the timebase advance + delay.dispatch while a break is active, so the victim's one-shot frame timers don't count down, expire, and get freed underneath it. The keyboard scan that follows tick in the frame IRQ still runs, so the monitor keeps input.
• Free-running VIA IRQ: on resume, a stuck VIA T1 interrupt flag is cleared so the edge-triggered interrupt controller latches the victim's next timer tick (otherwise a game clocking itself off VIA T1 in continuous mode hangs on resume).

Testing

Hardware-verified on the F256K2: break into games (incl. VIA-timer- and kernel-timer-driven), DOS, and SuperBASIC; inspect and resume with no visible disturbance.

Notes

• The reserved RAM banks are kept off-limits to SuperBASIC (its MaxPhysPage is lowered in that project).
• Native-mode NMI frame handling is out of scope (all current F256 software runs in emulation mode).