vmsig: management daemon, runtime endpoint lifecycle, roster, discovery, in-tree drivers, packaging

- core: runtime attach/detach of a per-endpoint adapter trio (runtime-safe add_adapter + vmsig_core_detach_endpoint, deferred reap)
- roster: VMSIG_EV_ROSTER + CAP_ROSTER, retained per-endpoint and replayed to late subscribers
- discovery: inotify trigger dir, vmid/endpoint slot allocator, host probe; vmsigd daemon with config + per-uid admission
- input driver and vgpu perception built in-tree; vgpu perception as a separate library
- memctx: own the supplied ro_fd (closed at detach)
- deb packaging: install rules, systemd unit, tmpfiles, default config

packaging/deb/prerm

@@ -0,0 +1,12 @@
+#!/bin/sh
+set -e
+case "$1" in
+remove|deconfigure)
+    if [ -d /run/systemd/system ]; then
+        systemctl disable --now vmsigd.service || true
+    fi
+    ;;
+upgrade|failed-upgrade)
+    ;;
+esac
+exit 0