- Primary display_control path now validates mode explicitly ('mirror'|'extend')
and returns an error instead of silently defaulting to extend.
Passes mode directly to binary (simpler, avoids redundant ternary).
- README: update set_display_layout bridge table row to correctly describe
display_control as primary and displayplacer as fallback.
- README: add one-time 'Build display_control Binary' section in Development
with xcode-select, build, test, and git commit steps.
86ea73bfbd