fix(View): change default zIndex from 0 to 'auto'

[owjsub]

Description

Changes the default zIndex of View from 0 to 'auto' to fix z-index layering issues with nested components.

Problem

Setting zIndex: 0 on a positioned element establishes a new local CSS stacking context, which isolates child elements with z-index values inside their parent View. This forces developers to manually add z-index to all nested Views just to make overlapping elements work correctly, as described in #2741.

Solution

By using zIndex: 'auto' instead, View no longer establishes a stacking context by default. This allows nested elements with z-index to participate in the parent stacking context and layer naturally relative to other positioned elements in the document tree, without requiring manual z-index management throughout the component hierarchy.

Changes

• Changed zIndex from 0 to 'auto' in View's default styles
• Added test to prevent regression and document the intent

Impact

This is a behavioral change that makes z-index work more intuitively and aligns better with React Native's behavior. Existing apps that don't rely on the stacking context isolation (uncommon) should see no difference. Apps that struggled with z-index layering will now work correctly without workarounds.

Closes #2741

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