docs: make reference metadata optional

.codex/skills/fesa-reference-models/SKILL.md

@@ -26,9 +26,9 @@ Read these first:
 2. Build a model inventory: smoke, analytical, patch test, benchmark, regression, and negative/invalid-input models.
 3. For each model, record `model_id`, purpose, verified requirements, analysis type, element type, material, boundary conditions, loads, expected quantities, tolerance, source, and status.
 4. Define `reference/<model-id>/` artifact bundle requirements.
-5. Require `model.inp`, `metadata.json`, `<model-id>_displacements.csv`, `<model-id>_reactions.csv`, `<model-id>_internalforces.csv`, `<model-id>_stresses.csv`, and `README.md` unless explicitly not applicable.
+5. Require `model.inp`, the applicable `<model-id>_displacements.csv`, `<model-id>_reactions.csv`, `<model-id>_internalforces.csv`, `<model-id>_stresses.csv`, and `README.md` unless explicitly not applicable. Treat `metadata.json` as optional.
 6. Define optional `<model-id>_strains.csv`, `<model-id>_energy_or_residual.csv`, and `<model-id>_<quantity>.csv` only when upstream acceptance criteria require them.
-7. Define metadata provenance, units, coordinate system, output requests, artifact status, reference_csv_schema_version, reference_csv_files, and limitations.
+7. Define metadata provenance, units, coordinate system, output requests, artifact status, reference_csv_schema_version, reference_csv_files, and limitations in the Reference Model Contract. If `metadata.json` exists, inventory it read-only and cross-check overlapping fields.
 8. Build a Coverage Matrix mapping requirement id, model id, compared quantity, FESA HDF5 dataset, reference CSV file, tolerance, verification method, and status.
 
 ## Output Contract
@@ -41,7 +41,7 @@ Produce or revise `docs/reference-models/<feature-id>-reference-models.md` with:
 - Model Record
 - Abaqus Input Requirements
 - Artifact Bundle Contract
-- Metadata JSON Contract
+- Reference Metadata Contract and optional `metadata.json` cross-check
 - Abaqus Reference CSV Requirements
 - Coverage Matrix
 - Artifact Acceptance Checklist
@@ -61,7 +61,8 @@ Produce or revise `docs/reference-models/<feature-id>-reference-models.md` with:
 
 - Every `must` requirement maps to at least one model and compared quantity.
 - `model.inp` stays within the supported Abaqus keyword subset or records an open issue.
-- `metadata.json` includes provenance, Abaqus version/source, units, coordinate system, tolerance, reference_csv_schema_version, and reference_csv_files.
+- The Reference Model Contract includes provenance, Abaqus version/source, units, coordinate system, tolerance, reference_csv_schema_version, and reference_csv_files.
+- An absent `metadata.json` is allowed and does not change readiness. A present file is read-only supplementary evidence; any conflict with the contract or stored artifacts remains visible as an upstream issue.
 - Missing required Abaqus reference CSV files keep the model at `needs-reference-artifacts`.
 
 ## Handoff