docs: make reference metadata optional

docs/reference-models/linear-static-3d-euler-beam-reference-models.md

@@ -187,16 +187,16 @@ These files shall not be generated, modified, renamed, corrected or restored by
 Harness. Their spaces and existing CAE report headers are legacy aliases, not a naming pattern for
 new models.
 
-Only for this approved legacy bundle:
+For this approved legacy bundle:
 
-- `metadata.json`: N/A
+- `metadata.json`: absent-allowed under the project-wide optional metadata policy
 - `README.md`: N/A
 - stress CSV: N/A because beam stress reference comparison is outside the approved V0 scope
 
-The approved design and this contract jointly replace those missing files by recording model ID,
-provenance, generator, source commit, units, coordinate systems, step/frame identity, logical CSV
-schema, exact inventory, tolerance policy and the stress N/A reason. Their absence therefore does
-not change the legacy record to `needs-reference-artifacts`.
+The approved design and this contract record model ID, provenance, generator, source commit,
+units, coordinate systems, step/frame identity, logical CSV schema, exact inventory, tolerance
+policy and the stress N/A reason. The optional metadata file's absence and the approved legacy
+README/stress exclusions therefore do not change the record to `needs-reference-artifacts`.
 
 ### Future reference bundles
 
@@ -207,7 +207,7 @@ marks a quantity N/A:
 reference/
   <model-id>/
     model.inp
-    metadata.json
+    metadata.json                     # optional
     <model-id>_displacements.csv
     <model-id>_reactions.csv
     <model-id>_internalforces.csv
@@ -215,14 +215,15 @@ reference/
     README.md
 ```
 
-CSV names are canonical `<model-id>_*.csv` names. `metadata.json` and `README.md` are mandatory.
-A quantity CSV may be omitted only when the upstream acceptance contract explicitly records N/A
-and gives its verification replacement. Missing required files keep that model at
-`needs-reference-artifacts`.
+CSV names are canonical `<model-id>_*.csv` names. `README.md` is mandatory for later bundles;
+`metadata.json` is optional. A quantity CSV may be omitted only when the upstream acceptance
+contract explicitly records N/A and gives its verification replacement. Missing required files
+or required Reference Model Contract provenance keep that model at `needs-reference-artifacts`.
 
-## Metadata JSON Contract
+## Reference Metadata Contract
 
-`metadata.json` is N/A only for `cantilever-beam-b33`. Every later bundle shall include at least:
+This document is the required source of truth for the following metadata. A later bundle may
+optionally duplicate it in `metadata.json` using at least this schema:
 
 ```json
 {
@@ -250,7 +251,8 @@ and gives its verification replacement. Missing required files keep that model a
 ```
 
 No agent may invent unknown provenance fields or mark a bundle ready merely because filenames
-exist.
+exist. An absent `metadata.json` is allowed. If the file exists, inventory it read-only and report
+any disagreement with this contract or stored artifacts as an upstream contract/provenance issue.
 
 ## Abaqus Reference CSV Requirements
 
@@ -365,7 +367,7 @@ Read-only inventory inspection on `2026-08-09` established the following pre-imp
 - every projected numeric field is finite and every row has the header arity;
 - generator, source commit, SI units, coordinates, step/increment/time and tolerance provenance are
   recorded in the approved design and this contract;
-- legacy `metadata.json`, `README.md` and stress CSV are accepted N/A exceptions;
+- absent `metadata.json` is allowed by project-wide policy; legacy `README.md` and stress CSV are accepted N/A exceptions;
 - no reference value was recalculated and no comparison was performed.
 
 Before an actual comparison, tooling must repeat all artifact checks, verify the exact Frame and