Implement link click navigation #156

Description#

Clicking an <a href="..."> element should navigate the browser to the target URL. This is the most fundamental navigation feature — without it, the browser is a single-page viewer.

Requirements#

• Hit-test mouse clicks against <a> elements in the render tree
• Extract the href attribute from the clicked anchor element
• Resolve relative URLs against the document's base URL (using the url crate)
• Initiate a new page load via the ResourceLoader
• Replace the current document in the browsing context with the newly loaded page
• Re-run style resolution, layout, and paint for the new document
• Handle target attribute basics: _self (default), _blank (ignore for now)
• Support <a> elements wrapping block-level content (not just inline text)
• Handle <area> elements inside <map> (stretch goal)

Acceptance Criteria#

• Clicking a link on a rendered HTML page loads and displays the linked page
• Relative URLs (e.g., href="/about") resolve correctly against the current page URL
• Absolute URLs (e.g., href="https://example.com") work
• Protocol-relative URLs (e.g., href="//example.com/page") work
• Links with href="" reload the current page
• Links with javascript: href are ignored (no crash)
• Unit tests for URL resolution and click-to-navigate flow