docs(steam): production plan for Deck client pass-through + shippable usbip host

Write design/steam-deck-passthrough-plan.md — the build plan to ship exact Steam
Deck pass-through from the Linux client (incl. the Steam + QAM buttons) plus a
virtual Deck on any Linux host. Key validated facts captured so the next session
doesn't re-investigate:

- Client capture is ALREADY correct: SDL3 maps Steam->Guide, QAM->Misc1; the
  client forwards BTN_GUIDE/BTN_MISC1; the host maps them to btn::STEAM/btn::QAM.
  Only precondition: Steam Input disabled on the client (the Decky UX).
- Shippable host transport = usbip + vhci_hcd (in-tree + signed everywhere, no
  module build, no MOK) — PROVEN on Bazzite: Steam promotes the usbip interface-2
  Deck (XInput slot + X-Box pad), identical to raw_gadget on SteamOS.
- Build steps: refactor steam_gadget.rs into shared Deck-logic + a transport
  trait; add the usbip transport (vendor-trim the usbip crate to drop rusb/libusb,
  in-process vhci attach); transport-select raw_gadget->usbip->UHID/DualSense;
  client leave-shortcut (controller chord + Ctrl+Alt+Shift+D); serial polish.

Also checks in the working usbip Deck PoC (packaging/linux/steam-deck-gadget/
usbip-poc/) for the next session to build on. Not pushed.

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

packaging/linux/steam-deck-gadget/usbip-poc/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "usbip-deck-poc"
+version = "0.0.0"
+edition = "2021"
+
+[[bin]]
+name = "usbip-deck-poc"
+path = "src/main.rs"
+
+[dependencies]
+usbip = "*"
+tokio = { version = "1", features = ["full"] }
+env_logger = "0.11"
+
+[profile.release]
+opt-level = 2