fix: avoid reallocating views on RCTDevLoadingView showMessage calls (#2762)

This PR is equivalent to the upstream PR
facebook#54608. Actually, the
upstream PR is a _little_ different, as I had to handle the new
`self->_dismissButton` that was addeed in
facebook#54445 as well. When I came
to handle `self->_dismissButton`, I took the opportunity to optimise the
`NSConstraintLayout` bit while I was there, so be ready for both of
those changes in `[email protected]`!

## Summary:

As this PR regards the dev loading view, this issue of course affects
only **debug** builds. The issue is that, each time a Metro progress
update triggers the `showMessage:withColor:withBackgroundColor:` method
on `RCTDevLoadingView`, we end up allocating a new `UIWindow` (iOS) or
`NSWindow` (macOS), rather than reusing the existing window stored on
`self->_window` from any previous progress updates. These unnecessary
allocations are particularly expensive on macOS, as I show below.

### Demo of the issue on macOS

On `react-native-macos`, the impact of this issue is dramatic (so much
so that **the Microsoft Office team disable `RCTDevLoadingView`** in
their Mac app). On the first connection to Metro (as first-time
connections can produce nearly 100 progress updates), we end up
allocating nearly 100 `NSWindow`s. Allocating `NSWindow`s is so
expensive that it blocks the UI thread and **adds 30 seconds to app
launch** (see 00:40 -> 01:15 of the below video clip).

What's more, as we can see in the view debugger, these `NSWindow`s
**never get released**, so they remain in the view graph and just leak
memory (see 01:15 -> 01:30 of the below video clip).

(This clip is from 1:15:00 of a
[livestream](https://www.youtube.com/live/amRWVbfbknM?si=KmDBXjrQXdxnmf1E&t=4500)
where I dug into this problem – sorry for the poor quality clips, that's
the best the servers allow me to download)


https://github.com/user-attachments/assets/65bb7c9b-dc18-4e54-8369-d0c611c59439

## Solution

Each of these views (the window, the container, and the label) need only
be allocated once. Thereafter, they can be modified. This PR adds
conditionals to lazily-allocate them, and reorders them in order of
dependency (e.g. the label is the most nested, so needs to be handled
first).

## Changelog:

[MACOS] [FIXED] - Avoid reallocating views on RCTDevLoadingView progress
updates

## Test Plan:

I am unable to run `RNTester` on the latest commit of the `main` branch;
I've tried [five different
versions](https://x.com/birch_js/status/1991129150728642679?s=20) of
Ruby, but the `bundle install` always fails. If someone could please
guide me how to get RNTester, Ruby, and Bundler happy, I'd be happy to
use that app to test it on `main`.

So my testing so far has been based on patching an existing
`[email protected]` app live on stream. This version of React
Native macOS has the exact same bug as current `main`, so it should be
representative.

Author: shirakaba

packages/react-native/React/CoreModules/RCTDevLoadingView.mm

@@ -128,33 +128,21 @@ - (void)showMessage:(NSString *)message color:(RCTUIColor *)color backgroundColo
     self->_window = [[UIWindow alloc] initWithWindowScene:mainWindow.windowScene];
     self->_window.windowLevel = UIWindowLevelStatusBar + 1;
     self->_window.rootViewController = [UIViewController new];
-#else // [macOS
-    self->_window = [[NSWindow alloc] initWithContentRect:NSMakeRect(0, 0, 375, 20)
-                                               styleMask:NSWindowStyleMaskBorderless | NSWindowStyleMaskFullSizeContentView
-                                                 backing:NSBackingStoreBuffered
-                                                   defer:YES];
-    [self->_window setIdentifier:sRCTDevLoadingViewWindowIdentifier];
-#endif // macOS]
 
-    self->_container = [[RCTUIView alloc] init]; // [macOS]
+    self->_container = [[UIView alloc] init];
     self->_container.backgroundColor = backgroundColor;
     self->_container.translatesAutoresizingMaskIntoConstraints = NO;
 
-    self->_label = [[RCTUILabel alloc] init]; // [macOS]
+    self->_label = [[UILabel alloc] init];
     self->_label.translatesAutoresizingMaskIntoConstraints = NO;
     self->_label.font = [UIFont monospacedDigitSystemFontOfSize:12.0 weight:UIFontWeightRegular];
     self->_label.textAlignment = NSTextAlignmentCenter;
     self->_label.textColor = color;
     self->_label.text = message;
 
-#if !TARGET_OS_OSX // [macOS]
     [self->_window.rootViewController.view addSubview:self->_container];
-#else // [macOS
-    [self->_window.contentView addSubview:self->_container];
-#endif // macOS]
     [self->_container addSubview:self->_label];
 
-#if !TARGET_OS_OSX // [macOS]
     CGFloat topSafeAreaHeight = mainWindow.safeAreaInsets.top;
     CGFloat height = topSafeAreaHeight + 25;
     self->_window.frame = CGRectMake(0, 0, mainWindow.frame.size.width, height);
@@ -175,6 +163,31 @@ - (void)showMessage:(NSString *)message color:(RCTUIColor *)color backgroundColo
       [self->_label.bottomAnchor constraintEqualToAnchor:self->_container.bottomAnchor constant:-5],
     ]];
 #else // [macOS
+    if (!self->_label) {
+      self->_label = [[RCTUILabel alloc] init]; // [macOS]
+      self->_label.translatesAutoresizingMaskIntoConstraints = NO;
+      self->_label.font = [UIFont monospacedDigitSystemFontOfSize:12.0 weight:UIFontWeightRegular];
+      self->_label.textAlignment = NSTextAlignmentCenter;
+    }
+    self->_label.textColor = color;
+    self->_label.text = message;
+
+    if (!self->_container) {
+      self->_container = [[RCTUIView alloc] init]; // [macOS]
+      self->_container.translatesAutoresizingMaskIntoConstraints = NO;
+      [self->_container addSubview:self->_label];
+    }
+    self->_container.backgroundColor = backgroundColor;
+    
+    if (!self->_window) {
+      self->_window = [[NSWindow alloc] initWithContentRect:NSMakeRect(0, 0, 375, 20)
+                                                styleMask:NSWindowStyleMaskBorderless | NSWindowStyleMaskFullSizeContentView
+                                                  backing:NSBackingStoreBuffered
+                                                    defer:YES];
+      [self->_window setIdentifier:sRCTDevLoadingViewWindowIdentifier];
+      [self->_window.contentView addSubview:self->_container];
+    }
+    
     // Container constraints
     [NSLayoutConstraint activateConstraints:@[
       [self->_container.topAnchor constraintEqualToAnchor:self->_window.contentView.topAnchor],