fix(client-linux,host): Deck video defaults to software decode + input-interception diagnostics

Video (Deck): the VAAPI zero-copy path renders corrupt/gray/washed-out on the
Deck — root-caused to Mesa >= 25.1 exporting radeonsi VCN decode surfaces TILED
(the Flatpak runtime's Mesa 26 drives both the decoder and GTK's GL, and GTK's
tiled-NV12 dmabuf import mishandles it; desktop Tier-1 validations ran distro
Mesa with linear export). `auto` now resolves to software on a Deck (clean,
correct-colour, easily handles 1280x800 HEVC); PUNKTFUNK_DECODER=vaapi still
forces the hw path, with the descriptor modifier dump + GSK_RENDERER as the
bisect levers. Also reserve extra_hw_frames=4 on the VAAPI decoder: the
presenter pins mapped surfaces past receive_frame, and the fixed pool recycling
a surface the renderer still samples is intermittent block corruption anywhere.

Input (Deck): with Steam Input ON for Punktfunk, SDL sees only Steam's virtual
X360 pad — the right trackpad arrives as a plain right stick and the left
trackpad/paddles/gyro not at all, silently. The client now checks once the
post-attach enumeration settles and raises a toast + warn naming the fix
(disable Steam Input for the shortcut). The host logs a one-shot warning when
InputPlumber is running (Bazzite default) since it can grab the virtual Deck
pad and re-emit it under a different identity.

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

crates/punktfunk-host/src/inject/linux/steam_controller.rs

@@ -276,12 +276,43 @@ impl DeckTransport {
     }
 }
 
+/// One-shot diagnostic: InputPlumber (shipped and enabled by default on Bazzite) hidraw-grabs
+/// controllers it decides to manage and re-emits them under a different identity — historically
+/// the Deck config re-emitted an Xbox Elite pad with the trackpads routed to a mouse target. If
+/// it grabs our virtual Deck, everything downstream of hid-steam looks wrong (trackpads surface
+/// as a stick/mouse, gyro vanishes) while punktfunk's own logs stay clean — so name the suspect
+/// up front. Best-effort process-name scan; no dependency on its D-Bus API.
+fn warn_if_inputplumber() {
+    use std::sync::atomic::{AtomicBool, Ordering};
+    static ONCE: AtomicBool = AtomicBool::new(true);
+    if !ONCE.swap(false, Ordering::Relaxed) {
+        return;
+    }
+    let running = std::fs::read_dir("/proc")
+        .ok()
+        .into_iter()
+        .flatten()
+        .flatten()
+        .any(|e| {
+            std::fs::read_to_string(e.path().join("comm")).is_ok_and(|c| c.trim() == "inputplumber")
+        });
+    if running {
+        tracing::warn!(
+            "InputPlumber is running on this host — if it manages the virtual Steam Deck pad, \
+             games see InputPlumber's re-emitted device instead (trackpads may arrive as a \
+             stick/mouse, gyro may vanish). Check `inputplumber devices` and exclude the \
+             virtual pad from management if inputs look remapped."
+        );
+    }
+}
+
 /// Open the best Steam-Input-promotable Deck transport available, in preference order:
 /// **`raw_gadget` (SteamOS validated fast-path) → `usbip`/`vhci_hcd` (universal, Secure-Boot-clean)
 /// → UHID (universal, but `Interface: -1` so Steam Input won't promote it).** Each rung degrades to
 /// the next on failure, so a host lacking the gadget modules still gets a *promotable* Deck via
 /// usbip, and one lacking both still gets a working (if non-promoted) UHID pad.
 fn open_transport(idx: u8) -> Result<DeckTransport> {
+    warn_if_inputplumber();
     use crate::inject::{steam_gadget, steam_usbip};
     // 1. raw_gadget — the validated SteamOS fast-path (default on there).
     if steam_gadget::gadget_preferred() {