Add virtual display for headless Depot runners (#721)

Depot macOS runners have no physical display, causing XCUITests to fail
with "Failed to activate application (current state: Running Background)".
This adds a small ObjC tool that creates a virtual display using the
private CGVirtualDisplay API before tests run.

scripts/create-virtual-display.m

@@ -0,0 +1,93 @@
+// Creates a virtual display on headless macOS (CI runners without a physical monitor).
+// Uses the private CGVirtualDisplay API from CoreGraphics.
+// The display stays alive as long as this process runs.
+//
+// Build: clang -framework Foundation -framework CoreGraphics -o create-virtual-display create-virtual-display.m
+// Usage: ./create-virtual-display &
+
+#import <Foundation/Foundation.h>
+#import <objc/runtime.h>
+
+// Private CoreGraphics classes (declared here since they're not in public headers)
+@interface CGVirtualDisplayMode : NSObject
+- (instancetype)initWithWidth:(unsigned int)width height:(unsigned int)height refreshRate:(double)refreshRate;
+@end
+
+@interface CGVirtualDisplayDescriptor : NSObject
+@property (nonatomic, copy) NSString *name;
+@property (nonatomic) unsigned int maxPixelsWide;
+@property (nonatomic) unsigned int maxPixelsHigh;
+@property (nonatomic) CGSize sizeInMillimeters;
+@property (nonatomic) unsigned int vendorID;
+@property (nonatomic) unsigned int productID;
+@property (nonatomic) unsigned int serialNum;
+@property (nonatomic, strong) dispatch_queue_t queue;
+@end
+
+@interface CGVirtualDisplaySettings : NSObject
+@property (nonatomic) unsigned int hiDPI;
+@property (nonatomic, strong) NSArray *modes;
+@end
+
+@interface CGVirtualDisplay : NSObject
+- (instancetype)initWithDescriptor:(CGVirtualDisplayDescriptor *)descriptor;
+- (BOOL)applySettings:(CGVirtualDisplaySettings *)settings;
+@property (nonatomic, readonly) unsigned int displayID;
+@end
+
+int main(int argc, const char *argv[]) {
+    @autoreleasepool {
+        unsigned int width = 1920;
+        unsigned int height = 1080;
+
+        // Verify the private classes exist
+        if (!NSClassFromString(@"CGVirtualDisplay")) {
+            fprintf(stderr, "ERROR: CGVirtualDisplay API not available on this system\n");
+            return 1;
+        }
+
+        // Create display mode
+        CGVirtualDisplayMode *mode = [[CGVirtualDisplayMode alloc] initWithWidth:width height:height refreshRate:60.0];
+        if (!mode) {
+            fprintf(stderr, "ERROR: Failed to create CGVirtualDisplayMode\n");
+            return 1;
+        }
+
+        // Configure descriptor
+        CGVirtualDisplayDescriptor *descriptor = [[CGVirtualDisplayDescriptor alloc] init];
+        descriptor.name = @"CI Virtual Display";
+        descriptor.maxPixelsWide = width;
+        descriptor.maxPixelsHigh = height;
+        descriptor.sizeInMillimeters = CGSizeMake(530, 300);
+        descriptor.vendorID = 0x1234;
+        descriptor.productID = 0x5678;
+        descriptor.serialNum = 0x0001;
+        descriptor.queue = dispatch_get_main_queue();
+
+        // Create virtual display
+        CGVirtualDisplay *display = [[CGVirtualDisplay alloc] initWithDescriptor:descriptor];
+        if (!display) {
+            fprintf(stderr, "ERROR: Failed to create CGVirtualDisplay\n");
+            return 1;
+        }
+
+        // Apply settings with display mode
+        CGVirtualDisplaySettings *settings = [[CGVirtualDisplaySettings alloc] init];
+        settings.hiDPI = 0;
+        settings.modes = @[mode];
+
+        BOOL ok = [display applySettings:settings];
+        if (!ok) {
+            fprintf(stderr, "ERROR: Failed to apply display settings\n");
+            return 1;
+        }
+
+        printf("Virtual display created: %ux%u@60Hz (displayID: %u)\n", width, height, display.displayID);
+        printf("PID: %d\n", getpid());
+        fflush(stdout);
+
+        // Keep alive so the display persists
+        dispatch_main();
+    }
+    return 0;
+}