feat(gamepad): classic Steam Controller backend — Linux UHID via hid-steam (N3)

The reserved GamepadPref::SteamController = 5 slot goes live: the same
hid-steam driver under the wired-SC identity (28DE:1102,
ID_CONTROLLER_STATE), UHID-only in v1 (no captured SC USB interface
layout, so no Steam-Input promotion — the pre-usbip Deck state;
acceptable for discontinued hardware).

Layout pinned against the kernel's ID_CONTROLLER_STATE table: 24-bit
buttons at 8..11 (low bits shared with the Deck; grips at 9.7/10.0 =
the Deck's L5/R5 positions; right-pad click 10.2; joystick click 10.6),
u8 triggers at 11/12, the joystick/left-pad MULTIPLEX at 16..20 (a
left-pad contact shadows the stick, like real hardware's lpad_touched
flag), right pad at 20..24. Mapping: wire left stick -> SC stick; wire
right stick -> right-pad coords + touched bit (the SC's camera surface —
the second-stick loss is inherent); PADDLE1/2 -> the two grips (natively,
masked out of the fold input); PADDLE3/4 + MISC1 -> the remap policy.
The SC parser has NO gamepad_mode gate, so no mode-entry pulse.

SteamDeckPad grew a SteamModel (open_model); ScProto/SteamCtrlManager;
pick_gamepad flips SteamController -> itself on Linux (replacing the
Xbox360 fold); SDL picker splits Valve PIDs (Deck 1205 stays SteamDeck,
SC 1102/1142 now declare SteamController).

Verified: .21 clippy -D warnings + 304/0 tests + on-box UHID smoke
(hid-steam binds 1102, BTN_A + right-pad ABS_RX land on evdev, no mode
pulse); .133 clippy -D warnings green.

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

crates/punktfunk-core/src/config.rs

@@ -156,9 +156,9 @@ pub enum GamepadPref {
     /// UHID DualShock 4 (kernel `hid-playstation`, ≥ 6.2) — lightbar, touchpad, motion, rumble. Like
     /// `DualSense` minus adaptive triggers / player LEDs / mute. Needs Linux UHID on the host.
     DualShock4,
-    /// UHID classic Steam Controller (Valve `28DE:1102`, kernel `hid-steam`) — dual trackpads, gyro,
-    /// two grip paddles, trackpad-only haptics. Needs Linux UHID. *(Reserved; its backend is not yet
-    /// built — currently folds to `Xbox360`; the Deck identity below is the implemented one.)*
+    /// UHID classic Steam Controller (Valve `28DE:1102`, kernel `hid-steam`) — one stick + dual
+    /// trackpads + two grip paddles. The wire right stick drives the right pad; a left-pad contact
+    /// shadows the stick (hardware multiplex). Needs Linux UHID.
     SteamController,
     /// UHID Steam Deck controller (Valve `28DE:1205`, kernel `hid-steam`) — full Deck gamepad incl.
     /// the four back grips (L4/L5/R4/R5), a right trackpad, and the IMU; re-grabbed by Steam Input