feat(macos): port RedBox 2.0 to AppKit

[Saadnajmi]

Summary

Targets 0.85-merge with two focused commits:

1. Sync the upstream React Native 0.85 RedBox 2.0 aggregate from
   [0.85] Backport RedBox 2.0 for iOS react/react-native#56640.
2. Port the RedBox 2.0 controller to AppKit without changing iOS, tvOS, or
   visionOS behavior.

This draft introduces the canonical RedBox2/AppKit implementation before the
0.86 fork merge. It does not rebase any existing stack branch.

Structural sync

The upstream aggregate adds the RedBox feature flags, extracted controllers,
C++ ANSI/error/JscSafeUrl support and tests, HMR support, CocoaPods ownership,
SwiftPM ownership, and feature-flag generated output.

The duplicate produced 20 conflicted paths: RCTRedBox.mm plus 19 generated
feature-flag files. The coordinator/controller resolution follows the existing
0.85 stable structural precedent, while generated outputs were regenerated
from ReactNativeFeatureFlags.config.js.

AppKit delta

The AppKit commit changes five files. It:

• enables redBoxV2IOS() selection on macOS while retaining the legacy
  controller when the flag is false;
• adds the NSTableView RedBox2 UI, AppKit footer buttons/key equivalents,
  NSPasteboard copy support, and sheet lifecycle;
• activates the target-version ANSI adapter on macOS;
• preserves the upstream iOS projection byte-for-byte after removing macOS
  blocks and additive clauses.

The old a87eee7 legacy-controller blob and stale ANSI adapter blob are
excluded. The target-version C++ parser, ErrorParser, HMR client, JscSafeUrl,
and later-compatible legacy-controller carve-outs remain authoritative.

Validation

Passed:

• exact S 47-path aggregate and bilateral conflict/provenance audit;
• generated feature flags and focused Jest tests (11/11);
• balanced [macOS] diff tags and iOS projection parity (5/5);
• CocoaPods and SwiftPM source ownership;
• RNTester Debug builds for macOS, iOS simulator, and visionOS simulator;
• all four affected RedBox sources compiled on all three platforms with zero
  source-attributed diagnostics;
• conflict-marker scan, git diff --check, graph/ref freeze, and clean
  Jujutsu working copy.

Native validation used a fresh no-local Git clone at exact AppKit commit
4ed3fb225714a6809cc69b8afdeffb88c0f8c80f. CocoaPods codegen ran first and
generated the required FBReactNativeSpec/AppSpecs headers.

The API comparison tool reports a pre-existing unrelated VirtualizedList delta
even for byte-identical base/candidate snapshots; the RedBox change contains no
JavaScript API delta.

Manual merge gate

Before merging this draft, manually enable redBoxV2IOS, trigger a JavaScript
error, and verify:

• the RedBox2 sheet and NSTableView rows/source/call stack;
• Copy and Option-Command-C copy through NSPasteboard;
• Reload and Command-R;
• Dismiss and Escape;
• sheet close/reopen lifecycle.

The feature defaults to false and RNTester has no native override, so this GUI
smoke was not performed by the validation-only executor.

Stack follow-up

After the manual GUI gate and explicit approval:

• recreate the 0.86/0.87 mainline subtree with merge-preserving Jujutsu
  rebases;
• add version-appropriate AppKit commits on top of 0.83 and 0.85 stable lines;
• leave 0.84 unchanged.

All current stack/* bookmarks remain unchanged by this PR.