Add css_loader module to the browser crate that collects CSS from the DOM: - Scan DOM for <style> elements (inline CSS) and <link rel="stylesheet"> (external) - Fetch external stylesheets via ResourceLoader with encoding detection - Resolve @import rules recursively (max depth 5 to prevent cycles) - Basic media attribute support (screen, all) - Type attribute validation (text/css default) - Graceful degradation: failed loads are silently skipped - Merge all rules into a single Stylesheet in document order 37 tests covering DOM scanning, classification, media matching, text collection, import resolution, error handling, and edge cases. Implements issue 3mhkt6vtv3q2g Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>