docs: make reference metadata optional

docs/io-definitions/linear-static-3d-euler-beam-io.md

@@ -302,9 +302,9 @@ Comparison 전에 다음 exact files가 존재하고 변경되지 않았음을
 Input은 `TYPE=B33`이어야 하고 trim한 CSV header는 아래 mapping 표와 정확히 일치해야
 한다. 각 projected row key는 unique하고 모든 numeric value는 finite여야 한다. 파일
 missing/type mismatch는 `needs-reference-artifacts`, header/value/identity mismatch는
-`schema-mismatch`이며 comparison은 시작하지 않는다. 이 legacy bundle에서
-`metadata.json`과 `README.md`는 N/A이고 파일을 rename, rewrite, zero-clamp 또는 보정하지
-않는다.
+`schema-mismatch`이며 comparison은 시작하지 않는다. `metadata.json` 부재는 프로젝트
+전역 정책에 따라 허용되고, `README.md`는 이 legacy bundle에서 N/A다. 파일을 rename,
+rewrite, zero-clamp 또는 보정하지 않는다.
 
 ### Header and component normalization
 

docs/io-definitions/linear-static-mitc4-shell-io.md

@@ -460,7 +460,7 @@ solver output nor a reference artifact.
 
 ### 7.1 S4/S4R portfolio separation
 
-- S4 and S4R use distinct `model_id`, input, metadata, and CSV bundles.
+- S4 and S4R use distinct `model_id`, input, Reference Model Contract records, and CSV bundles. An optional `metadata.json` does not define bundle identity.
 - FESA maps both source types to `FESA-MITC4`, but Abaqus S4 and S4R values are not
   expected to equal each other on a finite mesh.
 - Reference comparison always compares one FESA run with Abaqus rows generated from
@@ -484,7 +484,7 @@ The files were introduced by source commit
 zero-clamped, normalized, or repaired. A hash mismatch is
 `needs-reference-artifacts`, not permission to restore or change the bundle.
 
-The alias is `recognized-legacy-alias-needs-reference-metadata`. Before numerical
+The alias is `recognized-legacy-alias-needs-reference-contract`. Before numerical
 comparison, the Reference Model contract must supply or approve externally:
 
 - generation owner/date and exact provenance;
@@ -495,8 +495,8 @@ comparison, the Reference Model contract must supply or approve externally:
 - CSV schema version and tolerance policy.
 
 CSV lacks explicit step/frame/unit/coordinate columns. They map to
-`(Step-1,frame 0)`, global Cartesian, and documented units if and only if approved
-metadata confirms that exact interpretation. Absence or conflict is a
+`(Step-1,frame 0)`, global Cartesian, and documented units if and only if the approved
+Reference Model Contract confirms that exact interpretation. Missing contract data or a conflict is a
 schema/provenance failure; I/O Definition does not infer values.
 
 ### 7.3 Separate canonical S4R bundle
@@ -506,14 +506,15 @@ At least one S4R reference bundle is still required:
 ```text
 reference/<s4r-model-id>/
   model.inp
-  metadata.json
   <s4r-model-id>_displacements.csv
 ```
 
-`model.inp` must contain the approved subset and `TYPE=S4R`. Metadata records
-generator/version/provenance, unit system, coordinate system, model/step/frame,
-material/section/thickness, source element type, schema version, and tolerance
-policy. Optional reaction/stress artifacts remain nonblocking evidence.
+`model.inp` must contain the approved subset and `TYPE=S4R`. The approved Reference
+Model Contract records generator/version/provenance, unit system, coordinate system,
+model/step/frame, material/section/thickness, source element type, schema version, and
+tolerance policy. `metadata.json` is optional; if present, it is read-only supplementary
+evidence and every overlapping field is checked against the contract and stored artifacts.
+Optional reaction/stress artifacts remain nonblocking evidence.
 
 `<s4r-model-id>` is a Reference Model-owned schema variable, not an unresolved
 filename placeholder in this I/O contract.