fix: restore Linux keyboard injection

tests/linux_keyboard_x11_integration_test.cpp

@@ -0,0 +1,52 @@
+#include <X11/Xlib.h>
+
+#include <chrono>
+#include <cstdio>
+#include <thread>
+
+#include "keyboard_capturer.h"
+
+int main() {
+  Display* receiver = XOpenDisplay(nullptr);
+  if (!receiver) {
+    std::fprintf(stderr, "open display failed\n");
+    return 1;
+  }
+
+  const int screen = DefaultScreen(receiver);
+  Window window = XCreateSimpleWindow(receiver, RootWindow(receiver, screen),
+                                      0, 0, 100, 100, 0, 0, 0);
+  XSelectInput(receiver, window, KeyPressMask | KeyReleaseMask);
+  XMapWindow(receiver, window);
+  XSync(receiver, False);
+  std::this_thread::sleep_for(std::chrono::milliseconds(100));
+  XSetInputFocus(receiver, window, RevertToParent, CurrentTime);
+  XSync(receiver, False);
+
+  crossdesk::KeyboardCapturer keyboard;
+  // Exercise the scan-code fallback: the controller supplied no usable VK,
+  // but did preserve the set-1 scan code for the A key.
+  const int down = keyboard.SendKeyboardCommand(0, true, 0x1E, false);
+  const int up = keyboard.SendKeyboardCommand(0, false, 0x1E, false);
+
+  bool saw_down = false;
+  bool saw_up = false;
+  const auto deadline = std::chrono::steady_clock::now() +
+                        std::chrono::seconds(2);
+  while (std::chrono::steady_clock::now() < deadline &&
+         (!saw_down || !saw_up)) {
+    while (XPending(receiver) > 0) {
+      XEvent event{};
+      XNextEvent(receiver, &event);
+      saw_down = saw_down || event.type == KeyPress;
+      saw_up = saw_up || event.type == KeyRelease;
+    }
+    std::this_thread::sleep_for(std::chrono::milliseconds(10));
+  }
+
+  XDestroyWindow(receiver, window);
+  XCloseDisplay(receiver);
+  std::printf("down_ret=%d up_ret=%d saw_down=%d saw_up=%d\n", down, up,
+              saw_down, saw_up);
+  return down == 0 && up == 0 && saw_down && saw_up ? 0 : 2;
+}