Execute until ret

[Sightem]

This PR implements a new "Step Until Return" debugger command that allows the CPU to continue until it encounters a return instruction (RET, RETI, etc.) at or above the current stack level.

Behavior: When Step Until Return is activated, the debugger will:

1. Capture the current stack pointer level
2. Continue execution normally
3. Break when any return instruction (RET, RETI) is executed at or above the captured stack level
4. Display the debugger at the return instruction location

Keyboard shortcut: Shift+F9

Breakpoints/watchpoints cancel the command.