fix: restore menu-bar hiding on macOS 27

[thegeneralist01]

Summary

• keep the collapsed separator below macOS 27's half-screen discard threshold, allowing displaced items to enter the system overflow
• preserve the existing macOS 26-and-earlier behavior and avoid altering mixed-width displays by default; provide an opt-in for hiding on the narrowest display
• add a focused policy regression suite and document the new behavior

This independently validates the same root cause described in #382, with a small testable policy boundary and current-device verification.

Test plan

• sh tests/run-collapse-length-policy-tests.sh
• xcodebuild -project 'Hidden Bar.xcodeproj' -scheme 'Hidden Bar' -configuration Debug CODE_SIGNING_ALLOWED=NO build
• plutil -lint hidden/*.lproj/*.strings
• macOS 27 device test: click toggle produced separator widths 898 -> 22 -> 898