vmsig: a neutral signaling layer between sensors/input and controls

An epoll-driven, neutral transfer-event bus that connects sensors and input
actuators to one or more controls, bidirectionally. It owns the transfer context
and events — delivery order, priority, protocol-level timing, and an
interrupt-driven event model over fd sources (eventfd/timerfd/sockets) — and
stays agnostic to both the sensor/input drivers and the control.

What lives here:
- memctx: a coherent address-space context per endpoint — the guest address-space
  root paired with a pre-opened read-only RAM-region fd, with per-endpoint epoch
  invalidation and retained replay to late subscribers. Perception lives in
  out-of-tree sensor libraries that consume this datum read-only.
- exclusive-ownership leases for destructive resource classes (input, power,
  memory-write).
- write-signaled memory writes (MEMWRITE): an atomic write to guest memory routed
  through the seam under an exclusive lease, never a writable mapping.
- a host-management seam for VM lifecycle/status and a neutral input-injection
  command path.
- multi-VM endpoints; capability-gated, audited control authorization over an
  in-process or unix-socket transport.

Builds against headers only by default (a stub mode that exercises the seam
without a VM); armed builds link the real sensor/input libraries behind flags.

src/adapter/input/include/input.h

@@ -0,0 +1,18 @@
+#ifndef VMSIG_INPUT_H
+#define VMSIG_INPUT_H
+
+/* Private config of the input adapter (vmctl). cfg==NULL => stub mode. Armed mode
+ * (VMSIG_WITH_VMCTL) opens vmctl_open() and actuates for real. driver is an int so
+ * as not to pull vmctl.h into this header (values match VMCTL_DRIVER_*). */
+typedef struct {
+    int         stub;
+    int         driver;       /* 0=QMP, 1=UINPUT (see VMCTL_DRIVER_*)     */
+    const char* qmp_path;
+    const char* input_bus;
+    int         ptr_mode;
+} vmsig_input_cfg;
+
+/* Input event codes/contract are PUBLIC: vmsig_input / vmsig_input_kind in
+ * include/vmsig_event.h (external control encodes them into inln). No private duplicate. */
+
+#endif /* VMSIG_INPUT_H */