Add per-backend input debug visualization + round-trip view

Makes it possible to isolate an input->tool problem from a tool->game one in the
Controllers panel (under Debug details):
- which backend fed each slot is already shown via the per-pad source tag
  ("XInput #0" / "Steam Input") in the Incoming section;
- a new "Round-trip" table shows, per slot, the state we forwarded (the active
  backend's pad) next to what the game actually read back, highlighting matches.

For the round-trip, the XInput hook now echoes the state it returns to the game into
the status (protocol v9->v10: per-slot read_state in HookStatus).

Verified: hook_selftest (x64 + x86) asserts the hook records the game-read state;
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

@@ -175,6 +175,10 @@ int main()
 	check(block->status.rumble_left[0] == 0x8000 && block->status.rumble_right[0] == 0x4000,
 	      "status records rumble from XInputSetState");
 
+	// Round-trip: the hook echoes the state it returned to the game (for the
+	// Controllers panel's forwarded-vs-read view).
+	check(block->status.read_state[0].buttons == (kButtonA | kButtonB), "status records game-read state");
+
 	hook::remove_xinput_hooks();
 
 	std::printf(g_failures == 0 ? "SELFTEST PASS\n" : "SELFTEST FAILED (%d)\n", g_failures);