The AST conformance checker the contract promises does not exist

CONTRACT.md repeatedly delegates the invariants JSON Schema cannot express to a conformance checker, and that checker does not exist. CONTRACT.md:502 promises fixtures shipping with "a checker for the invariants JSON Schema cannot express — array-length equality, index correspondence, denormalised totals, deep-equality of a duplicated object". Nothing in scripts/ or tests/ implements it.

The effect is that several of the contract's load-bearing rules are enforced by nothing while both the prose and the schema descriptions tell an implementer they are machine-checked. A third-party wrapper runs npm run test:contract, sees green, and ships a fallback that disagrees with its own properties.

Found by an eleven-lens audit of the merged contract on 2026-09-03 (#14, squash f922dd7); each item was confirmed by a second agent told to refute it.

Two further points sharpen the work. The stated reason for not encoding the arity rules in the schemas is false: stats (rowCountFollowsStats) and chips (chipCountFollowsItems) already encode the structurally identical coupling by enumerating a closed tuple per size, so heatmap and delta-list can do the same. And check 4 of the validator asserts only the four root properties, every one of which the schemas already enforce, so it currently adds no coverage at all.

The work. Add an AST conformance check to scripts/validate-contract.mjs that runs a per-primitive invariant function over every valid sample, replace check 4's redundant assertions with a family-wide walk that fails on the three embed keys at any depth, encode the two arity couplings in the heatmap and delta-list schemas in the stats idiom, and correct every prose sentence that claims an enforcement which does not exist.

Start with scripts/validate-contract.mjs and run npm run test:contract to inspect the existing checks and valid samples. Add per-primitive invariant validation, replace check 4 with a family-wide walk, update the heatmap and delta-list schemas, and correct prose claims in CONTRACT.md and related schema descriptions. Done means the contract tests fail for the listed violations and pass for valid samples.