Add conformance fixtures for SPARQL Gremlin semantic search and analytics

[mdheller]

Add test-backed conformance fixtures for the implemented surfaces documented in the upstream convergence spec.

Acceptance criteria:

• SPARQL subset fixtures cover BGP, FILTER, OPTIONAL, ORDER BY, LIMIT, OFFSET, and CONSTRUCT.
• Gremlin subset fixtures cover V, E, hasLabel, has, out, in, both, outE, inE, values, valueMap, count, dedup, limit, and order.
• Semantic fixtures cover chunking, injected embeddings, cosine search, scoped search, and brain import/export.
• Analytics fixtures cover PageRank, warm-start PageRank, betweenness, and Louvain determinism.