The /api/models cache identity shipped in #7556 (release #7558) changed from a
stat identity to content fingerprints with volatile-key deny-lists, and the #7556
review flagged that this runtime contract had no project-level documentation.
Record the three source-fingerprint axes and their rationale, the one-directional
deny-list invariant, the stat-identity fallbacks (including the deep-tree
RecursionError path), the independent schema/version stamps, and the change
protocol; index the document from docs/CONTRACTS.md. Docs only.