docs(bench): inventory GDC SPB without adding an incompatible runner

[DecisionNerd]

Objective

Document GDC Semantic Publishing Benchmark while keeping its RDF/SPARQL workload outside the current Cypher benchmark implementation.

Requirements

• Document official scope, identities, protocol, and inventory-only rationale.
• State semantic prerequisites for a future executable adapter.
• Do not approximate SPARQL with Cypher or add product fallbacks.

Acceptance criteria

• Benchmark index includes SPB and current disposition.
• No executable SPB profile is advertised.
• Activation criteria are objective and testable.

BDD completion scenario

Given an operator reviews SPB
When its status is requested
Then the harness reports inventory-only with the semantic reason
And does not run an approximation.