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/vmhost/include/vmhost.h

@@ -0,0 +1,13 @@
+#ifndef VMSIG_VMHOST_H
+#define VMSIG_VMHOST_H
+
+/* Private config of the vmhost adapter (signaling's own QMP client).
+ * cfg==NULL or no qmp_path => stub mode (synthetic events, no QEMU).
+ * qmp_path given => armed: connect to QEMU's QMP socket ('@' prefix = abstract).
+ * No build flag needed — the client depends only on POSIX and its own code. */
+typedef struct {
+    int         stub;
+    const char* qmp_path;
+} vmsig_vmhost_cfg;
+
+#endif /* VMSIG_VMHOST_H */