Forward rumble back to the guest controller (both backends)

The XInput hook used to swallow XInputSetState; now it records the requested
left/right motor speeds into the status back-channel (protocol v8->v9: per-slot
rumble_left/right in HookStatus). Each frame the host reads them and, only on
change, drives the guest's actuator via the active backend:
- XInput: XInputSetState on the guest's slot (the open question is whether Steam's
  RPT virtual pad accepts vibration and routes it to the guest -- needs live RPT);
- Steam Input: SteamInput TriggerVibration on the slot's controller handle, with the
  XInput fallback for slots Steam isn't driving.

InputSource gains a set_rumble(slot,left,right) hook (default no-op) implemented by
both backends; SteamInputSource now tracks per-slot controller handles + which slots
it drives.

Verified: hook_selftest (x64 + x86) now asserts the hook records the rumble from
XInputSetState into the status; full build x64 + x86 clean; ctest x64 9/9, x86 3/3.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/input/xinput_source.cpp

@@ -39,4 +39,14 @@ void XInputSource::poll()
 	}
 }
 
+void XInputSource::set_rumble(int slot, std::uint16_t left, std::uint16_t right)
+{
+	if (slot < 0 || slot >= static_cast<int>(kMaxPads))
+	{
+		return;
+	}
+	XINPUT_VIBRATION v{left, right};
+	XInputSetState(static_cast<DWORD>(slot), &v);
+}
+
 } // namespace coop