Define and enforce a release artifact contract for populace-us (releases differ in files and manifest schema)

[PavelMakarchuk]

Problem

The three releases currently on policyengine/populace-us are mutually inconsistent:

release	build_manifest.json	release_manifest.json	sound_ecps_replacement_comparison.json
populace-us-2024-1abddeb-20260611	❌ missing	✅ (852 B, unversioned schema: release_id, country_id, artifacts, validation)	❌ missing
populace-us-2024-5da5a95-20260611	✅	✅ (168 KB, schema_version: 1)	✅
populace-us-2024-9f1260b-20260611	✅	✅ (2.7 KB, schema_version: 1)	✅

So a consumer iterating releases/ hits: a release with no build manifest at all, and two different release_manifest.json schemas (one without schema_version). The 5da5a95 vs 9f1260b size difference (168 KB vs 2.7 KB) also suggests the manifest contents aren't stable yet.

DESIGN.md makes "stage manifests are load-bearing" a binding process rule; the release directory is the most public manifest of all.

Proposal

1. Name the mandatory files for a release to count as published (suggest: build_manifest.json, release_manifest.json, sound_ecps_replacement_comparison.json).
2. Version the release-manifest schema and reject publishing on schema drift (a ten-line check in the publish step, in the spirit of the behavioral-contract-tests rule).
3. Either backfill or delete incomplete releases (1abddeb) so the listing is trustworthy.

Consumer

PolicyEngine/calibration-diagnostics Populace mode currently defends against this by filtering to releases that contain build_manifest.json, but the contract should live with the producer, not in every consumer.

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