feat(algo): add GDS_NODE2VEC (icebug/NetworKit node embeddings)

[zachwinter]

Summary

Second icebug-backed GDS function, following the pattern #30 established: CALL GDS_NODE2VEC('G') RETURN node, embedding runs NetworKit's Node2Vec over the projected graph and returns a LIST(FLOAT) embedding column (64 dimensions).

projected graph → InMemGraph CSR (same fwd/bwd scan as GDS_PAGE_RANK)
  → NetworKit::GraphR → NetworKit::Node2Vec(P=1, Q=1, walkLength=10, walksPerNode=5, D=64)
  → getFeatures() streamed out per node via ListVector

The bridge half is identical to gds_page_rank.cpp; the new plumbing is the output side — emitting a variable-length list per node through ListVector::addList + the list data vector, rather than a scalar. It builds its CSR with arrow::system_memory_pool() from the start (the mimalloc-under-dlopen issue — see #46).

Why this one next

Embeddings compose with the vector extension: LIST(FLOAT) casts to ARRAY(FLOAT, 64), which feeds CREATE_VECTOR_INDEX / HNSW. That makes GDS_NODE2VEC → cosine / HNSW a graph-native similarity pipeline with no external embedding step.

Testing

gds_node2vec.test: star graph, asserts every node gets a 64-dim embedding. Node2Vec is stochastic (unseeded), so the test asserts shape, not values.

Validated the semantic property separately: on a star graph, structurally-equivalent leaves embed at cosine ~0.997 to each other vs ~0.92 to the hub (via CAST(embedding AS FLOAT[64]) + array_cosine_similarity).

Notes / follow-ups

• Hyperparameters fixed for now (P=1, Q=1, L=10, N=5, D=64); a config object mirroring page_rank_config.h is the natural follow-up once the GDS surface question (per-algo functions vs a GDS_FUNCTION dispatcher) settles.
• Single-node-table graphs, same as GDS_PAGE_RANK.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I have added tests.

🤖 Generated with Claude Code

[zachwinter]

Rebased onto main now that #54 is in: this PR is now a single commit — GDS_NODE2VEC consuming the shared buildUndirectedCSR() helper (zero-copy from the projected graph's pinned CSR, scan fallback). Its private CSR/builder code is gone. 74/74 locally. The InMemGraph copy this review flagged is now dead in every GDS function.