fix(linux): drop implicit DRM master for card fds (#5286)

Co-authored-by: Kishi85 <41839133+Kishi85@users.noreply.github.com>

src/platform/linux/misc.h

@@ -5,6 +5,8 @@
 #pragma once
 
 // standard includes
+#include <fcntl.h>
+#include <filesystem>
 #include <unistd.h>
 #include <vector>
 
@@ -45,3 +47,24 @@ namespace dyn {
   void *handle(const std::vector<const char *> &libs);
 
 }  // namespace dyn
+
+namespace platf {
+  /**
+   * @brief Open a DRM card node and drop implicit DRM master, if any.
+   *
+   * Performs `open(path, flags | O_CLOEXEC)` and probes the resulting fd with
+   * `DRM_IOCTL_AUTH_MAGIC`. If the kernel implicitly handed us master, calls
+   * `drmDropMaster` and re-verifies before returning. Master check/drop failures
+   * are logged as warnings but do not fail the call: the caller still receives
+   * a usable fd.
+   *
+   * Callers should use this helper for any `/dev/dri/cardN` open so we never
+   * keep implicit master and block compositors from re-acquiring it on VT
+   * switches.
+   *
+   * @param path Path to the DRM card node (e.g. `/dev/dri/card0`).
+   * @param flags `open()` flags. `O_CLOEXEC` is always OR-ed in.
+   * @return A file descriptor on success, or `-1` if `open()` itself fails.
+   */
+  int open_drm_card_fd(const std::filesystem::path &path, int flags = O_RDWR);
+}  // namespace platf