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/README.md

@@ -0,0 +1,25 @@
+# usbip Deck PoC — the shippable virtual Deck for non-SteamOS Linux hosts
+
+Presents a real 3-interface USB Steam Deck (`28DE:1205`, controller on **interface 2**) over the
+usbip protocol via the `usbip` crate, so `vhci_hcd` can attach it **locally** — the Secure-Boot-clean,
+universal alternative to `dummy_hcd`+`raw_gadget` (which Bazzite/Fedora don't ship). Reuses the exact
+captured descriptors + feature contract from `crates/punktfunk-host/src/inject/linux/steam_gadget.rs`.
+
+**Validated live on Bazzite (2026-06-29):** `vhci_hcd` enumerates it, `hid-steam` binds it + reads the
+serial + makes the `Steam Deck` evdevs, stable (1 connect / 0 disconnect), and **Steam promotes it**
+(`Interface: 2 … reserving XInput slot 1`, X-Box pad emitted) — identical to the gadget on SteamOS.
+
+```sh
+cargo build --release           # glibc; needs GLIBC_2.34 (Bazzite has 2.42), libusb present/vendored
+# on the host (root for the last two):
+sudo modprobe vhci_hcd
+./usbip-deck-poc pressa &        # the usbip server (runs as a normal user, TCP 127.0.0.1:3240)
+sudo usbip attach -r 127.0.0.1 -b 0-0-0
+```
+
+See `design/steam-deck-passthrough-plan.md` for the production build plan (vendor-trim the crate to
+drop the `rusb`/libusb dep; in-process `vhci_hcd` attach to avoid the `usbip` CLI; transport-select
+`raw_gadget`→`usbip`→UHID). `usbip` crate API: a custom `UsbInterfaceHandler` —
+`get_class_specific_descriptor()` = the 9-byte HID descriptor; `handle_urb()` dispatches EP0
+`GET_DESCRIPTOR`(report) / HID `GET_REPORT`(=`feature_reply`) / `SET_REPORT`, and returns the 64-byte
+state report on the interrupt-IN endpoint.