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/Cargo.toml

@@ -89,6 +89,9 @@ tokio = { version = "1", features = ["rt", "rt-multi-thread", "net", "time"] }
 wayland-client = "0.31"
 wayland-protocols-wlr = { version = "0.3", features = ["client"] }
 wayland-protocols-misc = { version = "0.3", features = ["client"] }
+# `xdg-output` (zxdg_output_v1): the per-output *logical* geometry (post-scale size + global
+# position), used by the KWin fake_input backend to map absolute coordinates under display scaling.
+wayland-protocols = { version = "0.32", features = ["client"] }
 # Codegen for KDE's `zkde_screencast_unstable_v1` (vendored in `protocols/`): create a KWin
 # virtual output sized to the client's resolution and get its PipeWire node (KRdp's path).
 # `wayland-backend` is referenced by the generated interface tables.
@@ -119,6 +122,10 @@ ash = "0.38"
 # `libcuda.so.1` is dlopen'd at runtime (NOT link-time) so one Linux binary runs on NVIDIA
 # (zero-copy via CUDA) AND on AMD/Intel (VAAPI, no NVIDIA driver present) — see `zerocopy::cuda`.
 libloading = "0.8"
+# Vendored + trimmed `usbip` server core (no libusb) — presents a virtual Steam Deck over USB/IP
+# so the local `vhci_hcd` attaches it: the shippable, Secure-Boot-clean, Steam-Input-promotable
+# virtual-Deck transport on non-SteamOS hosts (`inject/linux/steam_usbip.rs`). See the crate's NOTICE.
+usbip-sim = { path = "vendor/usbip-sim" }
 
 [target.'cfg(target_os = "windows")'.dependencies]
 # Windows host backends. `windows` covers the Win32/CCD APIs the SudoVDA virtual-display backend