udev: connector hotplug, and rescue windows on an unplugged monitor

Monitors were probed once at startup, so plugging or unplugging one while
srdwm was running went unnoticed. A UdevBackend event source now watches for
the kernel's `change` uevent and reconciles the head list against a fresh
connector probe - forcing a re-probe rather than trusting cached status,
since on a hotplug the cache is exactly what has gone stale.

Removing a head tears down everything it owned: the wl_output global, its
place in the Space, its DRM framebuffers and dumb buffers (dropping the Rust
structs alone leaks the kernel-side objects, which matters when a cable is
plugged repeatedly), and any lock surface for it - otherwise
confirm_lock_if_presented would wait forever on a monitor that no longer
exists. New connectors go through the same bring_up_head path as startup, so
a monitor plugged in later is set up identically to one present at boot.
Heads are then repositioned left-to-right, since removing one shifts the
rest, and layer maps re-arranged so bars follow their moved output.

set_monitors rehomes windows stranded by the change, and main.rs re-queries
the whole monitor list on MonitorAdded/MonitorRemoved rather than applying
the single monitor in the event, because the others' positions move too.

The rehoming had a bug that unit tests missed and live testing caught.
It originally keyed off Window::monitor, but that field records the monitor
a window was *assigned* at creation, not where it is: add_window always sets
it from the primary monitor, so a window placed on the second monitor by a
rule - or dragged there - still reads monitor == 0. The field-only check
saw a valid id, skipped the window, and left it at coordinates that no
longer existed: invisible and unreachable. Found by unplugging a monitor out
from under a real xterm and watching it vanish from both heads. It now keys
off geometry, with a regression test that fails against the old logic.

Verified in the QEMU VM, booting with one connector and toggling the second
at runtime: plug in -> head added and rendering at its own resolution;
unplug -> head removed cleanly; and an xterm at global x=1500 survived its
monitor being unplugged, reappearing at x=680 (= min(1500, 1280-600)) with
its size intact. Writing to /sys/class/drm/<connector>/status changes the
connector but emits no uevent on this kernel, so the signal the kernel would
send is synthesized with `udevadm trigger`; the whole reaction path is
genuinely exercised.

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