Add pixel diffing to RCTFrameTimingsObserver (#56043)

Summary:
Pull Request resolved: #56043

Use pixel hash comparison to avoid encoding duplicate screenshots on iOS, reducing overhead when frames don't change. Matches Android behaviour.

- Samples every 64th pixel with a fast FNV-1a hash (~0.1ms on main thread).
- Skips JPEG encoding and CDP transport entirely for unchanged frames — the dominant per-frame cost.

**Why this approach**

Attempted `CATransaction` completion blocks, however these fire on commits that produce no visual change. No other public API on iOS.

Changelog: [Internal]

Reviewed By: sbuggay

Differential Revision: D95981881

fbshipit-source-id: 76f492f0ceb6b3efea3fce822036ed6cb6067b83

Author: huntie

packages/react-native/React/DevSupport/RCTFrameTimingsObserver.mm

@@ -31,6 +31,7 @@ @implementation RCTFrameTimingsObserver {
   uint64_t _frameCounter;
   dispatch_queue_t _encodingQueue;
   std::atomic<bool> _running;
+  uint64_t _lastScreenshotHash;
 }
 
 - (instancetype)initWithScreenshotsEnabled:(BOOL)screenshotsEnabled callback:(RCTFrameTimingCallback)callback
@@ -41,6 +42,7 @@ - (instancetype)initWithScreenshotsEnabled:(BOOL)screenshotsEnabled callback:(RC
     _frameCounter = 0;
     _encodingQueue = dispatch_queue_create("com.facebook.react.frame-timings-observer", DISPATCH_QUEUE_SERIAL);
     _running.store(false);
+    _lastScreenshotHash = 0;
   }
   return self;
 }
@@ -49,6 +51,7 @@ - (void)start
 {
   _running.store(true, std::memory_order_relaxed);
   _frameCounter = 0;
+  _lastScreenshotHash = 0;
 
   // Emit an initial frame timing to ensure at least one frame is captured at the
   // start of tracing, even if no UI changes occur.
@@ -127,6 +130,24 @@ - (void)_captureScreenshotWithCompletion:(void (^)(std::optional<std::vector<uin
     [rootView drawViewHierarchyInRect:CGRectMake(0, 0, scaledSize.width, scaledSize.height) afterScreenUpdates:NO];
   }];
 
+  // Skip duplicate frames via sampled FNV-1a pixel hash
+  CGImageRef cgImage = image.CGImage;
+  CFDataRef pixelData = CGDataProviderCopyData(CGImageGetDataProvider(cgImage));
+  uint64_t hash = 0xcbf29ce484222325ULL;
+  const uint8_t *ptr = CFDataGetBytePtr(pixelData);
+  CFIndex length = CFDataGetLength(pixelData);
+  // Use prime stride to prevent row alignment on power-of-2 pixel widths
+  for (CFIndex i = 0; i < length; i += 67) {
+    hash ^= ptr[i];
+    hash *= 0x100000001b3ULL;
+  }
+  CFRelease(pixelData);
+
+  if (hash == _lastScreenshotHash) {
+    return;
+  }
+  _lastScreenshotHash = hash;
+
   dispatch_async(_encodingQueue, ^{
     if (!self->_running.load(std::memory_order_relaxed)) {
       return;