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

[emmadesilva]

What changed (mirrors part A): RssFeedPage extends InMemoryPage lives next to its generator, resolves RssFeedGenerator from the container in compile() (rebind verified by test), registers in HydeCoreExtension::discoverPages() behind Features::hasRss() with the user-override skip check, and hides itself from navigation and the sitemap. GenerateRssFeed is removed, build:rss builds the registered route's page via StaticPageBuilder, and BuildTaskService now has no feature-gated tasks left (the Features import went with it).

Two deliberate divergences from the sitemap, both recorded in the epic:

• Verbatim filenames. The route key comes from hyde.rss.filename, and the old task wrote any configured name as-is — but the InMemoryPage extension allowlist would have turned feed.rss into feed.rss.html. RssFeedPage overrides identifierHasExplicitOutputExtension() to always use the configured filename as the literal output path, preserving old behavior for any extension (or none). I added a qualification to the epic's D2 open decision: the subclass override works as a first-party escape hatch, but the option-(b) call for user-land make() callers stays with PR 8.
• No URL guard on build:rss. The old RSS task never had one (unlike the sitemap task), so the explicit command still generates the feed regardless of feature conditions — each command keeps the semantics of the task it replaced.

Verification: RssFeedPageTest (14 tests) covers the three-way feature gating (URL, posts, config), custom and verbatim filenames, rebind, both D5 override paths end-to-end, and build-manifest/route:list presence; a realtime-compiler test serves feed.xml with application/xml. Full regression protocol ran clean against the part-A merge baseline: Unit failure lists identical, Feature lists identical after one real fallout (PageCollectionTest/RouteCollectionTest create posts, so the feed page appeared in their exact-collection assertions — they now disable RSS in setUp() like they already did the sitemap), FeatureHyde and Realtime Compiler suites fully pass, HydeStan reports no errors, and the CLI smoke shows both SitemapPage and RssFeedPage in route:list with build:rss producing valid feed XML. Release notes and UPGRADE.md were extended to cover both halves of the conversion.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (205fea8) to head (8199923).

Additional details and impacted files

@@                  Coverage Diff                  @@
##             v3/non-html-pages     #2530   +/-   ##
=====================================================
  Coverage               100.00%   100.00%           
- Complexity                1742      1747    +5     
=====================================================
  Files                      175       175           
  Lines                     4349      4359   +10     
=====================================================
+ Hits                      4349      4359   +10     

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