feat(iOS): Enable synchronous header config shadow state updates by default

[kmichalikk]

Description

This PR changes the default value for synchronousHeaderConfigUpdatesEnabled flag to true.

Changes

Updated flag default, native and JS defaults

Before & after - visual documentation

N/A

Test plan

See #3282

Checklist

• Included code example that can be used to test this change.
• For visual changes, included screenshots / GIFs / recordings documenting the change.
• For API changes, updated relevant public types.
• Ensured that CI passes

[Copilot]

Pull request overview

This PR flips the default for the experimental synchronousHeaderConfigUpdatesEnabled feature flag to true, enabling synchronous shadow state updates for stack header config on iOS by default (aligning JS, codegen defaults, and the native initial state).

Changes:

• Set the JS feature flag default synchronousHeaderConfigUpdatesEnabled to true.
• Update the Fabric codegen default for synchronousShadowStateUpdatesEnabled on RNSScreenStackHeaderConfig to true.
• Initialize the iOS native header config’s _synchronousShadowStateUpdatesEnabled to YES by default, and update the Fabric example flag to true.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/flags.ts	Changes the JS default for synchronousHeaderConfigUpdatesEnabled to true.
src/fabric/ScreenStackHeaderConfigNativeComponent.ts	Updates the codegen default for synchronousShadowStateUpdatesEnabled to true.
ios/RNSScreenStackHeaderConfig.mm	Sets the native initial default _synchronousShadowStateUpdatesEnabled to YES.
FabricExample/App.tsx	Enables the flag in the Fabric example configuration.

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