Conform selected legacy artifact decoding

[fruwe]

Summary

• Pin the implementation to specification cc4b0d734aa1c5953de75fb53b63e390a3b72761 and conformance 263644f951f342b0eeaa3aceef4877293d2d7c67.
• Support the closed decode_selected_migration_descriptor persistence-driver operation against its exact named artifact.
• Return each selected modern decoder's exact unsupported-format code when authentic legacy snapshot bytes omit that decoder's discriminator.
• Add focused regression coverage for the migration-descriptor resolver path and all selected artifact decoders.

Closes #76

Validation

• Python 3.11.16: 271 passed, 5 skipped; 278 conformance/profile tests passed
• Python 3.13.7: 271 passed, 5 skipped; 278 conformance/profile tests passed
• Ruff: passed
• Mypy: passed
• All five bundled schemas are byte-identical to the exact pinned specification
• git diff --check: passed

Scope

• No legacy converter or import support
• No execution-store scope or failure-registry changes
• No version bump, tag, publication, or release-workflow change