Add package READMEs

[ntucker]

Summary

• add installation and quick-start documentation for @reactive/silk
• document the platform-neutral APIs and entry points in @reactive/silk-core
• add React Native setup, theming, and component guidance for @reactive/silk-native

Verification

• ran npm pack --dry-run for all three publishable packages
• confirmed each package artifact includes its README
• ran whitespace validation

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Note

Low Risk
Documentation-only additions with no runtime, API, or security changes.

Overview
Adds npm-facing READMEs for @reactive/silk, @reactive/silk-core, and @reactive/silk-native with install, quick-start, theming, and entry-point guidance.

The web README covers SilkProvider, stylesheet options (styles.css vs styles.layer.css), and what the renderer includes. Core documents theme creation, recipes, and subpath exports. Native documents provider setup, shared semantic contracts, and the primitive set.

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[changeset-bot]

⚠️ No Changeset found

Latest commit: 4ae3133

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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