feat(gamepad): multi-controller support on the native plane

Host was already built for 16 pads; the blocker was every client
hard-coding pad 0. This lands the host-side + reference-client contract:

- input.rs: new wire kinds GamepadArrival=14 (declares a pad's type:
  code=GamepadPref byte, flags=pad) and GamepadRemove=13 (flags=seq<<24|pad,
  shares the snapshot seq space via encode/decode_gamepad_remove).
- pf-client-core/gamepad.rs: reworked from a single `open` pad to a
  slots: Vec<Slot> model — every forwarded controller gets a stable
  lowest-free wire index held for its lifetime, per-slot held/axis/touch/
  rumble state, GamepadArrival on open + GamepadRemove on close, and
  feedback routed back per wire index. Automatic forwards all real pads;
  a pin forces single-player.
- punktfunk1.rs: replaced the single-session PadBackend enum with a Pads
  router — per-pad kinds[]/owner[] arrays, lazily-created per-kind managers,
  pure route_decision keeping a live device in its manager across a kind
  change (no ghost/dup). Input thread seq-gates GamepadRemove (clears the
  pad_mask bit, resets rumble) and applies GamepadArrival kinds.
- inject linux/windows backends: add the two new no-op InputKind arms.

Native/session + default-Windows clients (both spawn punktfunk-session)
inherit this. 57 core + 33 client-core + 272 host tests green; clippy clean.

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

crates/punktfunk-host/src/inject/windows/sendinput.rs

@@ -301,6 +301,8 @@ impl InputInjector for SendInputInjector {
             InputKind::GamepadButton
             | InputKind::GamepadAxis
             | InputKind::GamepadState
+            | InputKind::GamepadRemove
+            | InputKind::GamepadArrival
             | InputKind::TouchDown
             | InputKind::TouchMove
             | InputKind::TouchUp => Ok(()),