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/inject/proto/steam_proto.rs

@@ -245,8 +245,11 @@ impl SteamState {
                 self.rpad_y = ((y as i32) - 32768) as i16;
             }
             RichInput::Motion { gyro, accel, .. } => {
-                self.gyro = gyro;
-                self.accel = accel;
+                // The wire carries DualSense-convention units (what every client capture emits); the
+                // Deck's hid-steam report wants 16 LSB/°·s + 16384 LSB/g, so rescale here.
+                let (g, a) = super::steam_remap::motion_wire_to_deck(gyro, accel);
+                self.gyro = g;
+                self.accel = a;
             }
             RichInput::TouchpadEx {
                 surface,
@@ -444,13 +447,15 @@ mod tests {
         assert_ne!(s.buttons & btn::RPAD_TOUCH, 0);
         assert_eq!(s.rpad_x, 32767); // 65535-32768
         assert_eq!(s.rpad_y, -32768); // 0-32768
+                                      // Motion is rescaled from the wire (DualSense) convention into Deck units (gyro ×16/20,
+                                      // accel ×16384/10000) — see steam_remap::motion_wire_to_deck.
         s.apply_rich(RichInput::Motion {
             pad: 0,
-            gyro: [1, 2, 3],
-            accel: [4, 5, 6],
+            gyro: [1000, -2000, 0],
+            accel: [10000, -5000, 0],
         });
-        assert_eq!(s.gyro, [1, 2, 3]);
-        assert_eq!(s.accel, [4, 5, 6]);
+        assert_eq!(s.gyro, [800, -1600, 0]);
+        assert_eq!(s.accel, [16384, -8192, 0]);
     }
 
     /// M3: the wire back-button bits map to the four Deck grips + QAM, and `TouchpadEx` routes the