Add runtime tablet/mouse toggle for relative pointer lock

[Fail-Safe]

Summary

• Keep absolute virtio-tablet as the desktop default, with stable QMP ids (omarchy-tablet / omarchy-mouse) so the pointer can hot-swap without restarting the VM.
• Add a Start Menu Relative mouse preference for launch, plus a menu-bar mouse control while the VM is running (the start menu dismisses at launch).
• Document that Cocoa full-grab is focused keyboard Super capture, not relative mouse lock — games need relative virtio-mouse before pointer lock works (fixes fix: Mouse Capture #144).

Test plan

• Launch with Relative mouse off: desktop trackpad pointing stays absolute; confirm virtio-tablet / id=omarchy-tablet in the QEMU command line.
• From the menu-bar mouse icon, switch to Game pointer, then verify a pointer-lock / relative-mouse app or game can capture the cursor (Ctrl-Alt releases).
• Switch back to Desktop pointer and confirm absolute pointing returns without restarting.
• Enable Relative mouse on the start menu, relaunch, and confirm the session starts on relative mouse.
• DEVELOPER_DIR=…Xcode… swift test --filter Pointer and macos/Tests/run-qemu-ssh-contract.test.sh

[Fail-Safe]

Closing — mid-session UI and QMP hotplug need a different approach before this is ready for review.