[codex] Support extended function key shortcut labels

[FreeTeaspoon]

Summary

• Add shortcut display labels for Carbon F13-F20 key codes so recorded Fn/function-key shortcuts no longer render as fn + ?.
• Add menu key equivalents for F13-F20.
• Cover extended function key display and menu-equivalent behavior in ShortcutCoreTests.

Validation

• xcodebuild test -project Snapzy.xcodeproj -scheme Snapzy -destination 'platform=macOS' -only-testing:SnapzyTests/ShortcutCoreTests CODE_SIGNING_ALLOWED=NO\n\n## Notes\n- An initial test run without CODE_SIGNING_ALLOWED=NO was blocked by the missing local Mac Development signing certificate for team XMHV5GH2Z7.\n