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>

tests/hook_selftest.cpp

@@ -170,6 +170,11 @@ int main()
 	check(block->status.get_state_calls[1].load(std::memory_order_relaxed) >= 1, "status counts slot 1 GetState");
 	check(block->status.get_caps_calls[0].load(std::memory_order_relaxed) >= 1, "status counts slot 0 GetCaps");
 
+	// Rumble forwarding: exercise_dll called XInputSetState(0, {0x8000, 0x4000}); the
+	// hook should have recorded it into the status for the host to forward to the guest.
+	check(block->status.rumble_left[0] == 0x8000 && block->status.rumble_right[0] == 0x4000,
+	      "status records rumble from XInputSetState");
+
 	hook::remove_xinput_hooks();
 
 	std::printf(g_failures == 0 ? "SELFTEST PASS\n" : "SELFTEST FAILED (%d)\n", g_failures);