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/control.in

@@ -0,0 +1,13 @@
+Package: vmsig
+Version: @VERSION@
+Section: admin
+Priority: optional
+Architecture: amd64
+Depends: @DEPENDS@
+Maintainer: @MAINTAINER@
+Description: VM signaling coherence daemon and host SI libraries
+ vmsig serves a unix-socket control plane over the signaling layer for the VMs it
+ discovers: lifecycle/state, coherent guest address-space context handoff, and arbitrated
+ input and memory-write actuation. Ships the daemon (vmsigd), the signaling library, the
+ host-side vgpu perception library, and a systemd unit. Configured via
+ /etc/vmsig/vmsigd.conf.