[3.x] Support pages with more output file extensions (Convert sitemap to a Hyde page)

[emmadesilva]

What changed:

• New SitemapPage extends InMemoryPage (Framework/Features/XmlGenerators/SitemapPage.php) whose compile() resolves SitemapGenerator from the container, so a container rebind swaps the output (verified by test — the generator is not final and has no constructor dependencies).
• Registered in HydeCoreExtension::discoverPages() behind Features::hasSitemap(), skipping when a page already claims the sitemap.xml route key (D5). Verified end-to-end that user pages win via both a booting() callback (visible to the skip check, which runs after booting callbacks) and a user HydeExtension (runs after core, replaces the page under the same collection key).
• GenerateSitemap removed (not deprecated — a kept task would double-generate) and dropped from BuildTaskService; build:sitemap now builds the registered route's page via StaticPageBuilder, falling back to new SitemapPage() so the explicit command still works when generate_sitemap is disabled. The no-base-URL failure now exits 1 instead of 3.

Epic decisions resolved and written back: D4's deferred plain-page-vs-subclass call went to the thin subclass — the command's fallback instantiation is exactly the concrete need the epic anticipated, mirroring BuildSearchCommand/DocumentationSearchIndex. Also recorded: the D2 allowlist confirmation for sitemap.xml, the D5 ordering semantics, and part B notes (mirror with RssFeedPage; the BuildTaskServiceUnitTest fixtures were migrated to GenerateBuildManifest so removing the RSS task won't churn them again).

Verification: New SitemapPageTest (11 tests) covers registration gating, build-manifest and route:list presence, self-exclusion, rebind, and both override paths; a realtime-compiler test confirms hyde serve returns the sitemap with application/xml. Full regression protocol ran clean: Unit and Feature suite FAILED lists are identical to the pre-change baseline (the ~40/51 remaining failures are the documented environmental PHP 8.5 asset/FeaturedImage set), FeatureHyde and Realtime Compiler suites fully pass, and HydeStan found no errors. One real fallout the suite caught: the monorepo's .env sets a site URL, so tests asserting exact default collections (PageCollectionTest, RouteCollectionTest, RouteListCommandTest) now saw the sitemap route — they disable the sitemap in setUp() to stay focused on collection mechanics. Release notes and an UPGRADE.md step (with the rebind/override migration paths) are included.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (64d074e) to head (c02bf70).

Additional details and impacted files

@@                  Coverage Diff                  @@
##             v3/non-html-pages     #2529   +/-   ##
=====================================================
  Coverage               100.00%   100.00%           
- Complexity                1738      1742    +4     
=====================================================
  Files                      175       175           
  Lines                     4340      4349    +9     
=====================================================
+ Hits                      4340      4349    +9     

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