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/linux/dualshock4.rs

@@ -367,6 +367,9 @@ pub struct DualShock4Manager {
     last_write: Vec<Instant>,
     /// Pad creation failed (e.g. /dev/uhid permissions) — warn once, drop events.
     broken: bool,
+    /// Fallback policy for the Steam back grips a client may send (the DS4 has no back-button HID
+    /// slot). `PUNKTFUNK_STEAM_REMAP=paddles=…`; default drop.
+    remap: crate::inject::steam_remap::RemapConfig,
 }
 
 impl Default for DualShock4Manager {
@@ -384,6 +387,7 @@ impl DualShock4Manager {
             last_led: vec![None; MAX_PADS],
             last_write: vec![Instant::now(); MAX_PADS],
             broken: false,
+            remap: crate::inject::steam_remap::RemapConfig::from_env(),
         }
     }
 
@@ -416,8 +420,12 @@ impl DualShock4Manager {
                 // Merge buttons/sticks/triggers, preserving touch + motion (those arrive on the
                 // rich-input plane and must survive a button-only frame).
                 let prev = self.state[idx];
+                // Steam back grips have no DS4 slot — fold them onto standard buttons per the
+                // configured policy (default drop) so they aren't silently lost.
+                let buttons =
+                    crate::inject::steam_remap::fold_paddles(f.buttons, self.remap.paddles);
                 let mut s = DsState::from_gamepad(
-                    f.buttons,
+                    buttons,
                     f.ls_x,
                     f.ls_y,
                     f.rs_x,