fix(display): skip mirror/extend if already in requested state

mirror_displays() now checks CGDisplayMirrorsDisplay() for each secondary
before calling CGBeginDisplayConfiguration — avoids a needless display
reconfiguration (and potential flicker) when already mirrored.
extend_displays() already had this check; no change needed there.

scripts/display_control.m

@@ -31,6 +31,17 @@ static int mirror_displays(void) {
 
     CGDirectDisplayID mainID = CGMainDisplayID();
 
+    // Idempotency: if every secondary is already mirroring mainID, nothing to do.
+    CGDisplayCount alreadyMirrored = 0;
+    for (CGDisplayCount i = 0; i < numOnline; i++) {
+        if (onlineDspys[i] != mainID && CGDisplayMirrorsDisplay(onlineDspys[i]) == mainID)
+            alreadyMirrored++;
+    }
+    if (alreadyMirrored == numOnline - 1) {
+        printf("Displays already mirrored.\n");
+        return 0;
+    }
+
     CGDisplayConfigRef config;
     CGError err = CGBeginDisplayConfiguration(&config);
     if (err != kCGErrorSuccess) {