Item 57: A checker for the applicability-in-source convention

An occurrence applying a schema to a case view must carry the applicability claim among its sources. The reason state depends on it: with the convention, an undercutter is an ordinary reason against the applicability claim and needs no attack primitive; without it, the enabling relation is wrong in a way the substrate cannot report.

The convention is stated and nothing enforces it. A checker over a reason ledger that fails an occurrence whose sources omit its own applicability claim is small, prospective, and exactly the shape checkers/contrib/ is open for.

Deliverable shape: a checker plus the claim it certifies, registered contributor-checked, or a house checker if the maintainer reads it. Acceptance check: python3 -m checkers.run accepts the entry, and the checker fails on a ledger containing one violating occurrence and on an empty ledger it was told to check. Context: .../2026-08-23-reason-representation/MEMO.md, Convention 1; .../2026-08-23-transition-certificates/README.md. Consumed by: any round treating applicability-in-source as enforced rather than declared. The transition round's certificate layer already does.