feat: Route opacity CSS transitions to the Android platform path

[MatiPl01]

Adds the Android arm of canRouteCSSProperty so opacity routes to the platform backend, behind the ANDROID_CSS_PLATFORM_TRANSITIONS static flag, and turns that flag on in the example apps.

The flag defaults to false, so nothing changes for users. Independent of the platform backend stack: until the Android hook is wired, apply() returns false on an empty std::function and every property falls back to the C++ loop, so this is a no-op whichever order it lands in.

Any TimeInterpolator can carry a curve, so unlike Apple every CSS easing routes; Kotlin rebuilds steps() and linear() stops from the same points the loop uses.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 65992368-7c77-4fc9-943a-fa5a03ef5033

📥 Commits

Reviewing files that changed from the base of the PR and between 66b8724 and 90815c5.

📒 Files selected for processing (1)

• packages/react-native-reanimated/Common/cpp/reanimated/CSS/utils/platform.cpp

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📝 Walkthrough

Summary by CodeRabbit

• New Features
  • Added support for routing Android opacity transitions through native platform transitions when the feature is enabled.
  • Android CSS transitions now accept all easing types for supported native transitions.
  • Added a feature flag to control this behavior, disabled by default in the main configuration.
• Bug Fixes
  • Unsupported CSS properties continue to use the standard animation path for reliable behavior.
• Configuration
  • Enabled the feature in the Android, macOS, and tvOS example applications for testing and demonstration.

Walkthrough

Adds the ANDROID_CSS_PLATFORM_TRANSITIONS static flag, enables it in example applications, and routes Android CSS transitions for opacity when the flag is enabled.

Changes

Android CSS platform transitions

Layer / File(s)	Summary
Transition flag configuration packages/react-native-reanimated/src/featureFlags/*, apps/*-example/package.json	Adds the flag with a default value of false and enables it in the Fabric, macOS, and tvOS examples.
Android transition routing packages/react-native-reanimated/Common/cpp/reanimated/CSS/utils/platform.*	Routes Android transitions for opacity when enabled. Updates the documentation for backend-specific routing and C++ fallback behavior.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• software-mansion/react-native-reanimated#10060: Introduces the Android CSS platform-transition implementation activated by this flag.
• software-mansion/react-native-reanimated#10061: Shares the Android transition flag and canRouteCSSProperty opacity routing.
• software-mansion/react-native-reanimated#10090: Connects Android platform transitions with opacity routing.

Suggested reviewers: pawicao, tjzel

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly states the main change: routing opacity CSS transitions through the Android platform path.
Description check	✅ Passed	The description accurately explains the Android routing logic, feature flag, default behavior, fallback path, and easing handling.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch @matipl01/android-css-opacity-routing-flag

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