feat(host/steam): M5 — fallback remap, motion rescale, degrade ladder

Keep the rich Steam inputs from silently dropping when the resolved backend
isn't the virtual hid-steam device, and fix a cross-device motion-scale bug.

- inject/proto/steam_remap.rs (new, pure + unit-tested):
  * motion_wire_to_deck — the wire carries DualSense-convention units (20 LSB/
    deg.s gyro, 10000 LSB/g accel — what every client capture emits), but the
    Deck's hid-steam report wants 16 LSB/deg.s + 16384 LSB/g. The Deck backend
    now rescales (gyro x16/20, accel x16384/10000): a real Deck<->Deck gyro/
    accel correctness fix (the DualSense/DS4 backends consume the wire 1:1).
  * fold_paddles + RemapConfig (PUNKTFUNK_STEAM_REMAP=paddles=drop|stickclicks|
    shoulders, default drop) — the DualSense + DS4 managers fold a client's back
    grips onto standard buttons rather than dropping them (those pads have no
    back-button HID slot; the uinput Xbox pad already exposes them as Elite
    paddles BTN_TRIGGER_HAPPY5-8).

- resolve_gamepad: a runtime degrade ladder — a UHID backend (DualSense / DS4 /
  Steam Deck) on a host where /dev/uhid isn't writable now falls back to the
  uinput Xbox 360 pad instead of a dead controller (the device-create would
  just fail). Separate from pick_gamepad's compile-time platform check, so the
  existing pick_gamepad tests are untouched.

- Delete the throwaway M0/M1 spike (src/bin/steam_uhid_spike.rs) — M2's
  #[ignore]d backend test subsumes its validation, and removing it frees
  steam_proto to reference steam_remap cleanly.

On-box backend test still green; workspace clippy/fmt/test green (incl. the new
steam_remap tests). Deferred as optional RemapConfig growth: gyro->mouse /
trackpad->stick synthesis on an Xbox target (no slot — documented drop today).
Not pushed.

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

crates/punktfunk-host/src/punktfunk1.rs

@@ -1917,6 +1917,36 @@ fn pick_gamepad(pref: GamepadPref, env: Option<&str>, linux: bool, windows: bool
     }
 }
 
+/// Runtime degrade for the Linux UHID backends (DualSense / DualShock 4 / Steam Deck): if
+/// `/dev/uhid` can't be opened for write *now*, fall back to the uinput X-Box 360 pad rather than a
+/// dead controller (the UHID device-create would just fail). Cheap — opens + drops the char device,
+/// no `UHID_CREATE2`, so no device is created. A no-op on non-Linux (those backends are UMDF/uinput).
+#[cfg(target_os = "linux")]
+fn degrade_if_no_uhid(chosen: GamepadPref) -> GamepadPref {
+    let needs_uhid = matches!(
+        chosen,
+        GamepadPref::DualSense | GamepadPref::DualShock4 | GamepadPref::SteamDeck
+    );
+    if needs_uhid
+        && std::fs::OpenOptions::new()
+            .write(true)
+            .open("/dev/uhid")
+            .is_err()
+    {
+        tracing::warn!(
+            wanted = chosen.as_str(),
+            "/dev/uhid not writable — falling back to the X-Box 360 pad"
+        );
+        return GamepadPref::Xbox360;
+    }
+    chosen
+}
+
+#[cfg(not(target_os = "linux"))]
+fn degrade_if_no_uhid(chosen: GamepadPref) -> GamepadPref {
+    chosen
+}
+
 /// Resolve the client's gamepad-backend preference (the env/logging shell around
 /// [`pick_gamepad`]). Always concrete — the `Welcome` reports what the session will drive.
 fn resolve_gamepad(pref: GamepadPref) -> GamepadPref {
@@ -1927,6 +1957,10 @@ fn resolve_gamepad(pref: GamepadPref) -> GamepadPref {
         cfg!(target_os = "linux"),
         cfg!(target_os = "windows"),
     );
+    // Runtime degrade (separate from the compile-time platform check above): the Linux UHID
+    // backends need `/dev/uhid` usable *now*, else creating the device just fails and the controller
+    // goes dead — fall back to the always-available uinput X-Box 360 pad instead.
+    let chosen = degrade_if_no_uhid(chosen);
     match pref {
         GamepadPref::Auto => {
             // The operator's env knob deserves a diagnostic when it didn't drive the