Brightness (Linux): fixes random built-in flag and stdout leak in DDC/CI detection (#2616)

DDC/CI results never set `builtin`, and FF_LIST_ADD does not zero the new
element, so external monitors were reported as built-in or external at
random. Set it to false, as the BSD DDC/CI backend does.

FF_SUPPRESS_IO() was declared inside the `if (ffddca_init)` block, so its
cleanup restored stdout before display detection ran and libddcutil trace
lines could reach the output, ahead of `--format json` too. Declare it at
function scope so it covers the whole detection, including the
FF_DISABLE_DLOPEN build.

Closes #2615

src/detection/brightness/brightness_linux.c

@@ -98,10 +98,12 @@ static const char* detectWithDdcci([[maybe_unused]] FFBrightnessOptions* options
     FF_LIBRARY_LOAD_SYMBOL_MESSAGE(libddcutil, ddca_free_any_vcp_value)
     FF_LIBRARY_LOAD_SYMBOL_MESSAGE(libddcutil, ddca_close_display)
 
+    // libddcutil may print trace messages to stdout during display detection, not only in ddca_init
+    FF_SUPPRESS_IO();
+
     #ifndef FF_DISABLE_DLOPEN
     FF_LIBRARY_LOAD_SYMBOL_LAZY(libddcutil, ddca_init)
     if (ffddca_init) {
-        FF_SUPPRESS_IO();
         // Ref: https://github.com/rockowitz/ddcutil/issues/344
         if (ffddca_init(nullptr, -1 /*DDCA_SYSLOG_NOT_SET*/, 1 /*DDCA_INIT_OPTIONS_DISABLE_CONFIG_FILE*/) < 0) {
             return "ddca_init() failed";
@@ -148,6 +150,7 @@ static const char* detectWithDdcci([[maybe_unused]] FFBrightnessOptions* options
                 brightness->max = max;
                 brightness->min = 0;
                 brightness->current = current;
+                brightness->builtin = false;
                 ffStrbufInitS(&brightness->name, display->model_name);
             }
             ffddca_close_display(handle);