feat(host/steam): shippable usbip/vhci_hcd virtual Deck + client leave-shortcuts

Steam Deck pass-through (design/steam-deck-passthrough-plan.md), code-complete +
all CI checks green on Linux + adversarially reviewed; on-glass validation pending:

- usbip/`vhci_hcd` virtual Deck transport (inject/linux/steam_usbip.rs) for
  non-SteamOS hosts (Bazzite/generic) — presents a real interface-2 USB Deck so
  Steam Input promotes it. In-process vhci attach (loopback OP_REQ_IMPORT handshake
  → sysfs attach) with a bounded `usbip`-CLI fallback; detach on drop.
- Backed by a vendored, libusb-free trim of the `usbip` crate
  (crates/punktfunk-host/vendor/usbip-sim, MIT + NOTICE; host/cdc/hid + rusb/nusb
  removed; interrupt-IN paced by bInterval).
- Selection ladder raw_gadget (SteamOS fast-path) → usbip (universal) → UHID,
  with PUNKTFUNK_STEAM_USBIP / PUNKTFUNK_USBIP_ATTACH knobs.
- Shared Deck descriptors + the 0x83/0xAE feature contract + a Steam-accepted
  serial consolidated into steam_proto.rs; the raw_gadget backend reuses them.
- Linux client leave-shortcuts: Ctrl+Alt+Shift+D + holding the escape chord
  (L1+R1+Start+Select) >=1.5s end the session (short press still exits
  fullscreen); the chord state resets across sessions.

Also bundles in-progress work already staged in the tree:
- host(kwin): xdg-output logical-geometry mapping so the KWin fake_input backend
  places absolute coordinates correctly under display scaling.
- docs: design/README index entries + design/controller-only-mode.md.

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

crates/punktfunk-host/vendor/usbip-sim/src/consts.rs

@@ -0,0 +1,122 @@
+use super::*;
+
+/// A list of known USB speeds
+#[derive(Copy, Clone, Debug)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub enum UsbSpeed {
+    Unknown = 0x0,
+    Low,
+    Full,
+    High,
+    Wireless,
+    Super,
+    SuperPlus,
+}
+
+/// A list of defined USB class codes
+// https://www.usb.org/defined-class-codes
+#[derive(Copy, Clone, Debug)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub enum ClassCode {
+    SeeInterface = 0,
+    Audio,
+    CDC,
+    HID,
+    Physical = 0x05,
+    Image,
+    Printer,
+    MassStorage,
+    Hub,
+    CDCData,
+    SmartCard,
+    ContentSecurity = 0x0D,
+    Video,
+    PersonalHealthcare,
+    AudioVideo,
+    Billboard,
+    TypeCBridge,
+    Diagnostic = 0xDC,
+    WirelessController = 0xE0,
+    Misc = 0xEF,
+    ApplicationSpecific = 0xFE,
+    VendorSpecific = 0xFF,
+}
+
+/// A list of defined USB endpoint attributes
+#[derive(Copy, Clone, Debug, FromPrimitive)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub enum EndpointAttributes {
+    Control = 0,
+    Isochronous,
+    Bulk,
+    Interrupt,
+}
+
+/// USB endpoint direction: IN or OUT.
+///
+/// Upstream re-exported `rusb::Direction`; vendored locally so this crate carries no libusb
+/// dependency. `UsbEndpoint::direction()` returns this, and `device.rs` matches on the variants.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub enum Direction {
+    /// Host → device (`bEndpointAddress` bit 7 clear).
+    Out,
+    /// Device → host (`bEndpointAddress` bit 7 set).
+    In,
+}
+
+/// Emulated max packet size of EP0
+pub const EP0_MAX_PACKET_SIZE: u16 = 64;
+
+/// A list of defined USB standard requests
+/// from USB 2.0 standard Table 9.4. Standard Request Codes
+#[derive(Copy, Clone, Debug, FromPrimitive)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub enum StandardRequest {
+    GetStatus = 0,
+    ClearFeature = 1,
+    SetFeature = 3,
+    SetAddress = 5,
+    GetDescriptor = 6,
+    SetDescriptor = 7,
+    GetConfiguration = 8,
+    SetConfiguration = 9,
+    GetInterface = 10,
+    SetInterface = 11,
+    SynchFrame = 12,
+}
+
+/// A list of defined USB descriptor types
+/// from USB 2.0 standard Table 9.5. Descriptor Types
+#[derive(Copy, Clone, Debug, FromPrimitive)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub enum DescriptorType {
+    /// DEVICE
+    Device = 1,
+    /// CONFIGURATION
+    Configuration = 2,
+    /// STRING
+    String = 3,
+    /// INTERFACE
+    Interface = 4,
+    /// ENDPOINT
+    Endpoint = 5,
+    /// DEVICE_QUALIFIER
+    DeviceQualifier = 6,
+    /// OTHER_SPEED_CONFIGURATION
+    OtherSpeedConfiguration = 7,
+    /// INTERFACE_POINTER
+    InterfacePower = 8,
+    /// OTG
+    OTG = 9,
+    /// DEBUG
+    Debug = 0xA,
+    /// INTERFACE_ASSOCIATION
+    InterfaceAssociation = 0xB,
+    /// BOS
+    BOS = 0xF,
+    // DEVICE CAPABILITY
+    DeviceCapability = 0x10,
+    /// SUPERSPEED_USB_ENDPOINT_COMPANION
+    SuperspeedUsbEndpointCompanion = 0x30,
+}