fix(macos): move Pin off Cmd-P and test menu chords

[yashranaway]

Summary

Implements the unblocked part of #160.

• Menu chords are a catalog in `MenuShortcuts.swift`. The Cocoa host applies that catalog instead of hard-coding modifiers in `buildMenu()`.
• Pin on Top is Cmd-Option-P. Cmd-P is no longer taken from Print.
• Snapshot stays Cmd-Shift-S on purpose.
• Cmd-, is not wired. Settings (macOS Settings window: shared preferences UI and Cmd+, shortcut #158) is parked.
• Protocol suite rejects duplicate chords and checks the Pin/snapshot/Settings reservations. P0 documents the chords.

Full GUI E2E of every menu item still wants a macOS session. The collision contract is tested without one.

Closes #160