feat: add native ring utilities

[e-simpson]

Summary

This PR adds browser-style native focus-ring utilities backed by React Native outline styles.

Behavior

• ring adds a 3-unit solid blue-500/50 browser-style ring.
• ring-{number} and ring-[{pixels}] set dynamic widths and inherit the same default blue when no explicit color is present.
• Explicit preset, custom, arbitrary, and opacity-modified ring colors override the default regardless of class order.
• ring-offset-* maps to native outlineOffset.

<View className="ring-2" />                    // default browser blue
<View className="ring-2 ring-red-500" />       // explicit red
<View className="dark:ring-white light:ring-black ring-2" />

Intentional Tailwind divergence

Tailwind CSS v4 implements rings with box shadows and defaults bare ring to 1px/currentColor. This package feature intentionally implements the requested browser-style focus treatment instead:

• native outlines are non-layout-shifting and match focus-ring semantics;
• the default remains the familiar blue browser treatment;
• rings and outlines share the same React Native properties, so they cannot render as two independent effects;
• inset rings and ring-offset colors have no direct outline equivalent and are not supported.

Validation

• bun run spec -- src/parser/rings.test.ts — 10 passed
• bun run check
• bun run format:check