Add W3C Referrer Policy support with all 8 policies: no-referrer, no-referrer-when-downgrade, origin, origin-when-cross-origin, same-origin, strict-origin, strict-origin-when-cross-origin (default), and unsafe-url. - New crates/net/src/referrer.rs: ReferrerPolicy enum, header/attribute parsing, and compute_referrer() for Referer header computation - ResourceLoader: tracks document URL and referrer policy, sets Referer header on all outgoing requests, updates policy from Referrer-Policy response headers - HTML integration: extracts <meta name="referrer"> from DOM, reads referrerpolicy attribute on <script>, <link>, and <img> elements - Element-level policy overrides document-level policy per spec - Strips fragment and userinfo from referrer URLs - HTTPS-to-HTTP downgrade correctly handled for strict/downgrade policies Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>