mze: add exit code / assertion support for automated testing

[oisee]

Description

For automated testing of Z80 binaries (e.g., book examples), mze needs a way to signal pass/fail from within the Z80 program. Currently there's no mechanism for a Z80 program to communicate a test result back to the host.

Proposed mechanisms

1. Exit code via port write: Write to a special I/O port (e.g., OUT ($FF),A) to set the process exit code. A=0 = pass, A≠0 = fail.

2. Halt detection: If execution reaches HALT instruction, exit cleanly (code 0). If execution hits --timeout, exit with error (code 1).

3. Memory assertion: After execution, check specific memory locations for expected values. Could be a flag like --assert 0x9000=0x42 that checks memory after the program terminates.

Context

We want make test-run in the book repo to:

1. Assemble each .a80 example with sjasmplus
2. Run the binary through mze with a timeout
3. Report pass/fail based on exit code

Currently mze always exits with code 0, even if the program crashes or loops forever (timeout just prints a message).

Current behavior

mze test.bin --timeout 1000; echo $?
# Always prints 0, even on timeout

Desired behavior

mze test.bin --timeout 1000; echo $?
# 0 if program terminated normally (RET/HALT)
# 1 if timeout reached
# N if program wrote N to exit port