feat: support arbitrary opacity values

[e-simpson]

Summary

This PR adds Tailwind-style arbitrary opacity values while preserving the package's existing named five-point scale.

Changes

Core Implementation

• opacity-[.67] and opacity-[0.67] map to React Native's normalized opacity value
• opacity-[67%] maps explicit percentages to the same native value
• Existing utilities such as opacity-65 remain unchanged
• Values outside React Native's 0 through 1 range are rejected with a development warning

Testing

• Covers raw decimal, integer boundary, whole-percentage, and decimal-percentage arbitrary values
• Covers invalid negative, over-range, and malformed arbitrary values
• Adds Babel transformation coverage proving opacity-[.67] compiles to opacity: 0.67

Documentation

• Adds named and arbitrary opacity examples to the Layout reference
• Documents the native range requirement

Examples

<View className="opacity-65" />       // { opacity: 0.65 }
<View className="opacity-[.67]" />    // { opacity: 0.67 }
<View className="opacity-[67%]" />    // { opacity: 0.67 }

Validation

• bun run spec (28 files; 1,059 passed, 1 skipped)
• bun run check
• bun run lint
• bun run format:check
• bun run build
• git diff --check