fix: Linux link errors and dbus shutdown warning

Add dialog_linux.c stubs for macOS-only native dialog symbols
(gui_native_dialog_result_ex_free et al). Remove invalid
dbus_connection_close on shared a11y bus connection.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mike-ward

nativebridge/a11y_linux.c

@@ -1254,7 +1254,8 @@ void gui_a11y_destroy(void) {
 
     dbus_connection_remove_filter(g_a11y_bus, a11y_filter,
         NULL);
-    dbus_connection_close(g_a11y_bus);
+    // dbus_bus_register makes the connection shared;
+    // shared connections must not be closed, only unref'd.
     dbus_connection_unref(g_a11y_bus);
 
     g_a11y_bus = NULL;

nativebridge/c_bindings.v

@@ -17,6 +17,7 @@ module nativebridge
 #flag linux -I/usr/lib/x86_64-linux-gnu/dbus-1.0/include
 #flag linux @VMODROOT/nativebridge/readback_linux.c
 #flag linux @VMODROOT/nativebridge/bookmark_stub.c
+#flag linux @VMODROOT/nativebridge/dialog_linux.c
 #flag linux @VMODROOT/nativebridge/portal_linux.c
 #flag linux -lGL
 #include "@VMODROOT/nativebridge/a11y_bridge.h"

nativebridge/dialog_linux.c

@@ -0,0 +1,71 @@
+// dialog_linux.c — Stubs for macOS-only native dialog functions
+// and shared result-free logic for Linux portal results.
+
+#include <stdlib.h>
+#include "dialog_bridge.h"
+
+static GuiNativeDialogResultEx dialog_stub_error(void) {
+    GuiNativeDialogResultEx result;
+    result.status = 2;
+    result.path_count = 0;
+    result.entries = NULL;
+    result.error_code = NULL;
+    result.error_message = NULL;
+    return result;
+}
+
+GuiNativeDialogResultEx gui_native_open_dialog_ex(
+    void* ns_window,
+    const char* title,
+    const char* start_dir,
+    const char* extensions_csv,
+    int allow_multiple
+) {
+    (void)ns_window; (void)title; (void)start_dir;
+    (void)extensions_csv; (void)allow_multiple;
+    return dialog_stub_error();
+}
+
+GuiNativeDialogResultEx gui_native_save_dialog_ex(
+    void* ns_window,
+    const char* title,
+    const char* start_dir,
+    const char* default_name,
+    const char* default_extension,
+    const char* extensions_csv,
+    int confirm_overwrite
+) {
+    (void)ns_window; (void)title; (void)start_dir;
+    (void)default_name; (void)default_extension;
+    (void)extensions_csv; (void)confirm_overwrite;
+    return dialog_stub_error();
+}
+
+GuiNativeDialogResultEx gui_native_folder_dialog_ex(
+    void* ns_window,
+    const char* title,
+    const char* start_dir,
+    int can_create_directories
+) {
+    (void)ns_window; (void)title; (void)start_dir;
+    (void)can_create_directories;
+    return dialog_stub_error();
+}
+
+void gui_native_dialog_result_ex_free(
+    GuiNativeDialogResultEx result
+) {
+    if (result.entries != NULL) {
+        for (int i = 0; i < result.path_count; i++) {
+            free(result.entries[i].path);
+            free(result.entries[i].data);
+        }
+        free(result.entries);
+    }
+    if (result.error_code != NULL) {
+        free(result.error_code);
+    }
+    if (result.error_message != NULL) {
+        free(result.error_message);
+    }
+}