feat(vdisplay/driver): DDC/CI against the virtual monitor fails fast

In exclusive topology the virtual display is the ONLY monitor on the desktop,
so monitor-control software (the Twinkle Tray / PowerToys PowerDisplay /
Monitorian class) aims its entire DDC traffic — brightness polls,
capabilities-string requests — at OUR monitor. There is no bus and no sink;
the only wrong answer is a slow one (a timeout-shaped failure occupies
win32k's physical-monitor path, serialized per monitor, for its full
duration). Register EvtIddCxMonitorI2CTransmit/Receive and answer every probe
with an immediate STATUS_NOT_SUPPORTED, making the virtual monitor the
cheapest thing on the "bus" to poll. (The receive DDI has no out-arg at the
callback — refusing synchronously refuses the whole transaction.) EDID needs
no equivalent: the OS serves descriptor queries from the blob supplied at
monitor creation without calling the driver.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packaging/windows/drivers/pf-vdisplay/src/callbacks.rs

@@ -375,3 +375,30 @@ pub unsafe extern "C" fn device_io_control(
     // SAFETY: `request` is the framework-provided WDFREQUEST; `control::dispatch` completes it exactly once.
     unsafe { crate::control::dispatch(request, ioctl_code) };
 }
+
+/// DDC/CI transmit toward the virtual monitor — refuse INSTANTLY (stall-immunity: DDC fail-fast).
+///
+/// In exclusive topology the virtual display is the ONLY monitor on the desktop, so
+/// monitor-control software (the Twinkle Tray / PowerToys PowerDisplay / Monitorian class) aims
+/// its entire DDC traffic — brightness polls, capabilities-string requests — at THIS monitor.
+/// There is no bus and no sink here; the only wrong answer is a slow one (a timeout-shaped
+/// failure occupies win32k's physical-monitor path, serialized per monitor, for its full
+/// duration). Registering the pair pins every probe's cost to one immediate
+/// `STATUS_NOT_SUPPORTED`. EDID needs no equivalent: the OS serves descriptor queries from the
+/// blob supplied at monitor creation without calling the driver.
+pub unsafe extern "C" fn monitor_i2c_transmit(
+    _monitor: iddcx::IDDCX_MONITOR,
+    _p_in: *const iddcx::IDARG_IN_I2C_TRANSMIT,
+) -> NTSTATUS {
+    crate::STATUS_NOT_SUPPORTED
+}
+
+/// DDC/CI receive from the virtual monitor — same contract as [`monitor_i2c_transmit`]. (The
+/// receive DDI has no out-arg at the callback — data would flow back through an async completion;
+/// a synchronous failure return refuses the whole transaction, which is exactly the point.)
+pub unsafe extern "C" fn monitor_i2c_receive(
+    _monitor: iddcx::IDDCX_MONITOR,
+    _p_in: *const iddcx::IDARG_IN_I2C_RECEIVE,
+) -> NTSTATUS {
+    crate::STATUS_NOT_SUPPORTED
+}

packaging/windows/drivers/pf-vdisplay/src/entry.rs

@@ -89,6 +89,11 @@ extern "C" fn driver_add(_driver: WDFDRIVER, mut init: PWDFDEVICE_INIT) -> NTSTA
     cfg.EvtIddCxMonitorSetGammaRamp = Some(callbacks::set_gamma_ramp);
     cfg.EvtIddCxMonitorAssignSwapChain = Some(callbacks::assign_swap_chain);
     cfg.EvtIddCxMonitorUnassignSwapChain = Some(callbacks::unassign_swap_chain);
+    // DDC fail-fast (stall-immunity): monitor-control software polls the virtual monitor's DDC —
+    // in exclusive topology it is the ONLY monitor. These answer every probe with an immediate
+    // STATUS_NOT_SUPPORTED instead of whatever slow path an unregistered interface takes.
+    cfg.EvtIddCxMonitorI2CTransmit = Some(callbacks::monitor_i2c_transmit);
+    cfg.EvtIddCxMonitorI2CReceive = Some(callbacks::monitor_i2c_receive);
     cfg.EvtIddCxDeviceIoControl = Some(callbacks::device_io_control);
 
     // SAFETY: init is the framework device-init; cfg is fully populated + sized. (Links IddCxStub.)

packaging/windows/drivers/pf-vdisplay/src/lib.rs

@@ -35,6 +35,7 @@ use wdk_sys::NTSTATUS;
 // NTSTATUS codes the driver returns (wdk-sys doesn't surface all of these as constants).
 pub(crate) const STATUS_SUCCESS: NTSTATUS = 0;
 pub(crate) const STATUS_NOT_IMPLEMENTED: NTSTATUS = 0xC000_0002u32 as NTSTATUS;
+pub(crate) const STATUS_NOT_SUPPORTED: NTSTATUS = 0xC000_00BBu32 as NTSTATUS;
 pub(crate) const STATUS_NOT_FOUND: NTSTATUS = 0xC000_0225u32 as NTSTATUS;
 pub(crate) const STATUS_INVALID_PARAMETER: NTSTATUS = 0xC000_000Du32 as NTSTATUS;
 pub(crate) const STATUS_BUFFER_TOO_SMALL: NTSTATUS = 0xC000_0023u32 as NTSTATUS;