Ruby 3.4 support and Yoast-inspired SEO features

[marcosgz]

Summary

• Ruby 3.4 compatibility — updated dependencies and fixed offenses
• Configurable max URLs per sitemap — config.max_links (default 50k, set to 1k for Yoast-style performance)
• Priority/changefreq deprecation — config.emit_priority and config.emit_changefreq flags to omit elements Google ignores
• XSL stylesheets — human-readable HTML display when sitemaps are opened in a browser, with built-in templates for both URL sets and sitemap indexes
• Rack middleware — SiteMaps::Middleware serves sitemaps with X-Robots-Tag: noindex, follow, proper Content-Type, Cache-Control, XSL endpoints, and 301 redirects for page 0/1 URL normalization
• robots.txt helper — SiteMaps::RobotsTxt generates the Sitemap: directive
• Search engine ping — SiteMaps::Ping notifies Bing after generation (Google deprecated their endpoint), integrated with Runner and notification system
• Sitemap index lastmod propagation — index lastmod now reflects the most recent child sitemap instead of Time.now
• External sitemap registration — external_sitemap DSL adds third-party sitemaps to the index
• URL filtering callbacks — url_filter blocks to exclude or modify URLs before they enter the sitemap
• README rewrite — full documentation of all features with table of contents

Test plan

• bundle exec rspec — 360 examples, 0 failures
• bundle exec rubocop — 0 offenses
• Verify XSL rendering by opening a generated sitemap XML in a browser
• Test middleware with a Rack app serving sitemaps at configured paths
• Verify SiteMaps::RobotsTxt.render output
• Verify SiteMaps::Ping.ping with webmock stubs
• Verify URL filtering excludes/modifies URLs in process blocks
• Verify external sitemaps appear in the sitemap index