fix: resolve relative fasmg executable paths

[sit-d]

Summary

• Resolve a user-supplied relative --fasmg path before launching the assembler.
• Preserve existing absolute-path and executable discovery behavior.

Verification

• python3 -m py_compile scripts/*.py
• python3 scripts/gen_all.py --lint (46 regex patterns passed)
• Focused validator fixture: a relative ./stub-fasmg path ran all 30 pass/fail cases successfully.

Notes

• The focused fixture substitutes assembler exit statuses; validation against a real fasmg binary was not run.