Allow a root Atom feed to declare one source section

[forkwright]

Finding

Typikon's global Atom template always consumes Zola's site-wide feed pages. A consumer cannot
declare that one publication section, such as writing/ or journal/, owns canonical root
/atom.xml while retaining root-feed autodiscovery and the shared Typikon template.

This is already a two-consumer substrate need:

• Ardent Tools carries a local templates/atom.xml fork so only writing/ publishes.
• Ardent Leatherworks needs the same contract for journal/.

#31/#40 deliberately preserved Zola's site-wide page set while fixing the empty-feed case.
#92/#171 corrected <updated> calculation without adding membership scope. Neither owns this
setting.

Desired correction

Add an optional consumer setting such as:

[extra]
feed_source_section = "journal/_index.md"

For the global feed, resolve that section and emit only its dated, feed-eligible direct pages.
Preserve current site-wide behavior when the setting is absent and preserve Zola's supplied page
set for native section/taxonomy feeds. A configured scope must own the feed title, subtitle,
alternate link, membership, and <updated> value. Missing or empty configured scopes should fail
by name instead of silently falling back to site-wide freshness.

Expose include_in_feeds in the applicable closed page schemas so consumers can exclude one entry
without forking the template.

Acceptance

1. A fixture with two Journal entries and a newer dated product emits exactly the two Journal IDs
   at root /atom.xml.
2. Editing Terms or product content leaves the scoped Atom file byte-identical.
3. Editing an older Journal entry with a newer updated value advances both the entry and feed
   timestamps.
4. include_in_feeds = false excludes a scoped entry.
5. Missing or empty configured sections fail with the configured path in the error.
6. An unset scope preserves Preserve Zola's page context in the global Atom feed #31's site-wide behavior.
7. Ardent Tools can remove its Atom shadow and configure writing/_index.md; Leatherworks can
   configure journal/_index.md; both consumer gates pass.

[forkwright]

Satisfied at origin/main abe82f11b954528e0cfcfa975efda9ca0d7aca1e, landed by PR #185. Closing on
a tree citation rather than on the PR's description of itself.

• templates/atom.xml:28-70 implements feed_source_section, and fails closed rather than falling
  back: a missing or empty declared section throws (:33-36, :52-54) instead of silently emitting
  the site-wide feed, which was the failure mode the issue's Finding described.
• :49 gates entries on include_in_feeds.
• schemas/page.core.schema.json:49-52 and schemas/journal-entry.schema.json:22 expose the field,
  so a consumer declaring it is validated rather than trusted.
• ci/check-scoped-atom-feed.py is the fixture.

Acceptance item 7 — consumer migration — is deliberately not counted here. It is work in the consumer
repositories (ardent-tools/ardent-tools-site#169, forkwright/ardent-site#37), not in Typikon, and
holding this issue open for it would make Typikon's own tracker report a Typikon defect that does not
exist.