test(canopy-rules): grow cargo xtask rules check with JDM-level assertion tests

Adding a structured test-fixture format for decision tables (input JSON → expected output JSON) would let us lock in the code mapping at ruleset-validation time, independent of the Rust-side integration tests. cargo xtask rules check could grow a test mode that walks fixtures and asserts emitted fields.

Extracted from tanf-denial-reason-code-from-jdm.adoc § Potential Improvements.