[Feature]: Implied relationship — checkMetadataFeatureSemanticSpecialization (blocked on #256)

[antoineatstariongroup]

Problem

checkMetadataFeatureSemanticSpecialization (KerML §8.4.2 Specialization category) is the only
in-scope semantic constraint not computed by SysML2.NET.Semantics.

Its OCL calls MetadataFeatureExtensions.ComputeSyntaxElementOperation, which is a stub throwing
NotSupportedException.

Blocked by

#256 — Implementation of MetadataFeatureExtensions.ComputeSyntaxElementOperation.

#322 (model-level expression evaluation engine) is NOT a prerequisite, despite the matching
subject matter.

Work once unblocked

• Add MetadataFeatureSemanticSpecializationRule in SysML2.NET.Semantics/Implied/Rules/,
  registered in SysML2.NET.Semantics/Extensions/ServiceCollectionExtensions.cs.
• Ground the Relationship kind with hypha:spec-citation before implementing — sibling constraints
  in this family use FeatureTyping, Subsetting, or kind-by-target, and the OCL does not say which.

Acceptance criteria

• The rule is registered and applies only when isSemantic() holds
• Unit tests cover the positive case, the non-semantic case, and the null-argument guard
• Existing suites stay green

Context

Closes the last in-scope gap in #338. See .team-notes/implied-relationship-layer-plan.md §12.