Test Windows VM removal with setgid shared directory

[AlexR1712]

This adds a removal regression test to the branch behind omacom/omarchy#12264.

Dockurr's sharing service can set /shared to mode 2777. On current quattro, prepare_caller_mounts runs chmod 0700, which leaves the setgid bit in place. The subsequent exact-mode check sees 2700, so removal stops before deleting the VM disk.

The test sets the shared directory to 2777, runs prepare_caller_mounts, and calls __priv_remove. It checks that removal deletes the disk data, keeps the shared files, and leaves the shared directory at 700. The namespace test calls prepare_caller_mounts explicitly because the production removal path calls it through privileged assert_mounts_safe.

With this test added, windows-vm-compose-test.sh exits 1 on untouched quattro at prepare_caller_mounts and passes on the head of omacom#12264. windows-vm-launcher-test.sh, windows-vm-test.sh, and git diff --check pass too.

I also reproduced the error on Omarchy 4.0.4-1 after a successful omarchy-windows-vm install. The Windows ISO download was still retrying, so I applied the chmod 2777 /shared operation from Dockurr's running /run/samba.sh inside the container. omarchy-windows-vm remove then failed with the reported message and left the VM data intact. Clearing setgid let the same removal command succeed.

I ran ./test/all. The changed compose test passed, but the aggregate suite exited 1 with 14 failing test files. Several tests wrote directly to this host's quota-limited /tmp; others require local packages or services. The failures include the existing windows-vm-mount-boundary-test.sh.