feat(windows-host): STEP 4 (3/n) — host pf_vdisplay backend (talks to the new driver)

The host can now drive the new pf-vdisplay IddCx driver instead of SudoVDA. Compiles
clean on BOTH Windows (cargo check -p punktfunk-host green) and Linux (cfg(windows)-gated,
main CI unaffected); adversarially reviewed (no blockers, lockstep with the driver).

- new vdisplay/pf_vdisplay.rs: cloned from the proven sudovda.rs, repointed to
  pf_vdisplay_proto — interface GUID 70667664 (not e5bcc234), IOCTL 0x900-0x905 (not the
  gappy 0x800/0x888/0x8FF), AddRequest/AddReply/RemoveRequest/SetRenderAdapterRequest
  (bytemuck Pod, not the GUID-keyed AddParams), a u64 session_id monitor key (not a minted
  GUID), and a single IOCTL_GET_INFO handshake that HARD-asserts protocol_version (vs
  SudoVDA two-IOCTL best-effort). Full MGR/linger/refcount/teardown lifecycle preserved.
- reuses sudovda.rs backend-neutral CCD/DXGI helpers (set_active_mode, isolate/restore_
  displays_ccd, resolve_gdi_name, resolve_render_adapter_luid, MON_GEN/CURRENT_MON_GEN,
  SavedConfig) — widened to pub(crate), not duplicated.
- vdisplay::open()/probe() select the backend: PUNKTFUNK_VDISPLAY=pf|sudovda forces one;
  default auto-detects (prefer pf-vdisplay if its interface enumerates, else SudoVDA stays
  the shipping fallback).

Notes: SET_RENDER_ADAPTER is tolerated as the driver returns NOT_IMPLEMENTED today (STEP 4
tail); the cross-MGR wait_for_monitor_released only paces sudovda's MGR (benign until
IDD-push lands on pf-vdisplay, STEP 6 — documented in-code). On-glass "monitor appears at
WxH@Hz" gate is next.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/vdisplay.rs

@@ -529,9 +529,15 @@ pub fn open(compositor: Compositor) -> Result<Box<dyn VirtualDisplay>> {
     }
     #[cfg(target_os = "windows")]
     {
-        // Windows has a single virtual-display backend (SudoVDA); the compositor arg is moot.
+        // Two virtual-display backends: the new pf-vdisplay IddCx driver (pf_vdisplay_proto) and the
+        // shipping SudoVDA fallback. The compositor arg is moot on Windows. PUNKTFUNK_VDISPLAY overrides;
+        // default auto-detects (prefer pf-vdisplay if its driver interface is present).
         let _ = compositor;
-        Ok(Box::new(sudovda::SudoVdaDisplay::new()?))
+        if windows_use_pf_vdisplay() {
+            Ok(Box::new(pf_vdisplay::PfVdisplayDisplay::new()?))
+        } else {
+            Ok(Box::new(sudovda::SudoVdaDisplay::new()?))
+        }
     }
     #[cfg(not(any(target_os = "linux", target_os = "windows")))]
     {
@@ -540,6 +546,22 @@ pub fn open(compositor: Compositor) -> Result<Box<dyn VirtualDisplay>> {
     }
 }
 
+/// Pick the Windows virtual-display backend. `PUNKTFUNK_VDISPLAY=pf|pf-vdisplay|pfvd` forces the new
+/// pf-vdisplay IddCx driver; `=sudovda|sudo` forces the shipping SudoVDA driver; anything else (the
+/// default) auto-detects, preferring pf-vdisplay if its device interface is enumerable.
+#[cfg(target_os = "windows")]
+fn windows_use_pf_vdisplay() -> bool {
+    match std::env::var("PUNKTFUNK_VDISPLAY")
+        .ok()
+        .as_deref()
+        .map(str::trim)
+    {
+        Some("pf") | Some("pf-vdisplay") | Some("pfvd") => true,
+        Some("sudovda") | Some("sudo") => false,
+        _ => pf_vdisplay::is_available(),
+    }
+}
+
 /// Readiness probe for `compositor`: is it up and able to create a virtual output *right
 /// now*? A session-bringup script polls this (via `punktfunk-host probe-compositor`) to gate
 /// on actual readiness instead of racing the compositor with a blind sleep.
@@ -560,7 +582,11 @@ pub fn probe(compositor: Compositor) -> Result<()> {
     #[cfg(target_os = "windows")]
     {
         let _ = compositor;
-        sudovda::probe()
+        if windows_use_pf_vdisplay() {
+            pf_vdisplay::probe()
+        } else {
+            sudovda::probe()
+        }
     }
     #[cfg(not(any(target_os = "linux", target_os = "windows")))]
     {
@@ -608,6 +634,8 @@ mod kwin;
 #[cfg(target_os = "linux")]
 mod mutter;
 #[cfg(target_os = "windows")]
+pub(crate) mod pf_vdisplay;
+#[cfg(target_os = "windows")]
 pub(crate) mod sudovda;
 #[cfg(target_os = "linux")]
 mod wlroots;