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

src/adapter/memctx/include/memctx.h

@@ -8,8 +8,10 @@ typedef struct {
     int         stub;       /* 1 => synthetic kcr3/RO-fd (spine without a VM)           */
     const char* ram_path;   /* armed: path to guest RAM backing (NOT published outward) */
     uint64_t    low;        /* below-4G split (vmie_win32_open / locator.low)           */
-    int         ro_fd;      /* >=0 => infra supplied a pre-sealed RO-fd (policy);       */
-                            /* <0  => default: open(ram_path, O_RDONLY) / stub-memfd    */
+    int         ro_fd;      /* >=0 => infra hands a pre-sealed RO-fd (policy); OWNERSHIP */
+                            /*        TRANSFERS to the adapter (closed in close()) — the */
+                            /*        caller dups first if it must keep its own copy.    */
+                            /* <0  => default: open(ram_path, O_RDONLY) / stub-memfd     */
 } vmsig_memctx_cfg;
 
 /* Max SRC bytes per atomic gva_write (bounds the worker POD slot; mc_req header + src