Add data_url module to the url crate: - Base64 decoder (RFC 4648): standard alphabet, whitespace stripping, padding - Data URL parser: extract MIME type, charset, base64/percent-encoded payload - Default MIME type text/plain;charset=US-ASCII when omitted Integrate with ResourceLoader in browser crate: - Handle data: URLs in both fetch() and fetch_url() without network access - Classify decoded content into Html/Css/Image/Other resource types - Decode text resources using charset from data URL parameters 44 tests covering base64 decoding, data URL parsing, MIME extraction, percent decoding, and ResourceLoader integration. Implements issue 3mhkt7m45472x Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>