feat(windows): pf-vdisplay CLEAR_ALL — reap orphaned virtual monitors on startup

The "5-6 stale monitors that never tear down" failure (also seen with SudoVDA):
an orphan from a crashed/killed previous host lingers because the driver watchdog
is kept reset by a still-pinging new session, so it never fires for the orphan.

- Driver (pf-vdisplay control.rs): new IOCTL_CLEAR_ALL (0x804) -> tear down every
  monitor. A pf-vdisplay extension; SudoVDA returns invalid for it (ignored), so
  the host can issue it unconditionally.
- Host (vdisplay/sudovda.rs): send IOCTL_CLEAR_ALL once on startup (best-effort)
  to reap orphans before creating ours; and surface a failing keepalive PING (the
  old `let _ =` swallowed it, masking a lost control handle).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/punktfunk-host/src/vdisplay/sudovda.rs

@@ -53,6 +53,9 @@ const IOCTL_ADD: u32 = ctl(0x800);
 const IOCTL_REMOVE: u32 = ctl(0x801);
 const IOCTL_SET_RENDER_ADAPTER: u32 = ctl(0x802); // == 0x0022_2008
 const IOCTL_GET_WATCHDOG: u32 = ctl(0x803);
+/// pf-vdisplay extension (NOT in SudoVDA): tear down every virtual monitor. Sent once on host startup
+/// to reap monitors orphaned by a crashed/killed previous host. SudoVDA returns invalid (ignored).
+const IOCTL_CLEAR_ALL: u32 = ctl(0x804);
 const IOCTL_DRIVER_PING: u32 = ctl(0x888);
 const IOCTL_GET_VERSION: u32 = ctl(0x8FF);
 
@@ -722,10 +725,20 @@ unsafe fn create_monitor(device: isize, mode: Mode, watchdog_s: u32) -> Result<M
         let stop_t = stop.clone();
         let pinger = thread::spawn(move || {
             let h = HANDLE(device_raw as *mut c_void);
+            let mut warned = false;
             while !stop_t.load(Ordering::Relaxed) {
                 let mut none: [u8; 0] = [];
-                unsafe {
-                    let _ = ioctl(h, IOCTL_DRIVER_PING, &[], &mut none);
+                match unsafe { ioctl(h, IOCTL_DRIVER_PING, &[], &mut none) } {
+                    Ok(_) => warned = false,
+                    // A persistently failing PING means the cached control handle went invalid — the
+                    // driver watchdog will then tear the monitor down mid-session. Surface it once
+                    // (the old `let _ =` swallowed it, which masked exactly this during the bad-state churn).
+                    Err(e) => {
+                        if !warned {
+                            tracing::warn!("SudoVDA keepalive PING failed (control handle lost?): {e:#}");
+                            warned = true;
+                        }
+                    }
                 }
                 thread::sleep(interval);
             }
@@ -848,6 +861,16 @@ fn mgr_ensure_device(g: &mut Mgr) -> Result<isize> {
         3
     };
     tracing::info!("SudoVDA watchdog timeout {}s", g.watchdog_s);
+    // Reap monitors orphaned by a crashed/killed previous host instance before we create ours.
+    // pf-vdisplay honors IOCTL_CLEAR_ALL; SudoVDA returns invalid (ignored). Without it an orphan
+    // lingers until the driver watchdog fires — but a still-pinging new session keeps resetting that
+    // watchdog, so orphans could accumulate (the "5-6 stale monitors that never tear down" failure).
+    {
+        let mut none: [u8; 0] = [];
+        if unsafe { ioctl(device, IOCTL_CLEAR_ALL, &[], &mut none) }.is_ok() {
+            tracing::info!("cleared orphaned virtual monitors on host startup");
+        }
+    }
     let raw = device.0 as isize;
     g.device = Some(raw);
     Ok(raw)

packaging/windows/vdisplay-driver/pf-vdisplay/src/control.rs

@@ -30,6 +30,10 @@ const IOCTL_ADD: u32 = ctl(0x800);
 const IOCTL_REMOVE: u32 = ctl(0x801);
 const IOCTL_SET_RENDER_ADAPTER: u32 = ctl(0x802);
 const IOCTL_GET_WATCHDOG: u32 = ctl(0x803);
+/// pf-vdisplay extension (NOT in SudoVDA): tear down every monitor. The host issues this on startup to
+/// reap monitors orphaned by a crashed/killed previous host instance. SudoVDA returns invalid for it
+/// (harmlessly ignored), so the host can send it unconditionally.
+const IOCTL_CLEAR_ALL: u32 = ctl(0x804);
 const IOCTL_PING: u32 = ctl(0x888);
 const IOCTL_GET_VERSION: u32 = ctl(0x8FF);
 
@@ -112,6 +116,10 @@ pub extern "C-unwind" fn device_io_control(
             IOCTL_SET_RENDER_ADAPTER => do_set_render_adapter(request, input_len),
             IOCTL_GET_WATCHDOG => do_get_watchdog(request, output_len, &mut bytes),
             IOCTL_PING => NTSTATUS::STATUS_SUCCESS,
+            IOCTL_CLEAR_ALL => {
+                disconnect_all_monitors();
+                NTSTATUS::STATUS_SUCCESS
+            }
             IOCTL_GET_VERSION => do_get_version(request, output_len, &mut bytes),
             _ => NTSTATUS::STATUS_INVALID_DEVICE_REQUEST,
         }