Wires up macOS 27 support

[samhenrigold]

Closes #25.

On macOS 27, window-based MenuBarExtra no longer uses the status item button's target/action. Presentation moved to AppKit's private "expanded interface session" machinery. A click begins an NSStatusItemExpandedInterfaceSession and notifies the item's expandedInterfaceDelegate (SwiftUI's WindowMenuBarExtraBehavior), which shows/hides the window.

This broke pretty much everything in this package.

Fix

All macOS 27 stuff is isolated in NSStatusItem+ExpandedInterfaceSession.swift and guarded by responds(to:) checks; macOS 26 and earlier is totally unchanged.

• Present: call _beginExpandedInterfaceSession: directly. I found that _requestExpandedInterfaceSession is pretty flaky, AppKit drops begins whose event timestamp predates the last session end, so repeated programmatic presentation silently fails.
• Dismiss: cancel expandedInterfaceSession which is the same call SwiftUI makes.
• State: isMenuBarExtraPresented reads the session on 27, button state otherwise.
• Disable: detach the delegate while disabled (stashed weakly, restored on re-enable) so a click never begins a session.
• The resign-key failsafe cancels the session instead of closing the window, which would orphan the session.