feat: CSR adjacency index for native graph traversal

[jja725]

Summary

Adds a CSR (Compressed Sparse Row) adjacency index that enables O(1) neighbor lookup for graph traversal, replacing SQL join-based expansion with direct pointer-chasing. This is the foundation for wiring up the LanceNativePlanner placeholder with a real native execution path.

Inspired by GraphAr's CSR-in-Parquet approach (Apache incubator), adapted for Lance's columnar format.

What's included

• CsrIndex — in-memory CSR structure with:

  • neighbors(vertex_id) — O(1) neighbor lookup via offset array
  • degree(vertex_id) — O(1) out-degree
  • bfs(start, max_hops) — k-hop BFS traversal returning vertices by distance
  • shortest_path(start, end) — BFS-based unweighted shortest path
  • to_record_batch() / neighbors_to_record_batch() — Arrow serialization for persisting as Lance datasets

• CsrIndexBuilder — construct CSR from:

  • Individual add_edge(src, dst) calls
  • Arrow RecordBatch with src_id/dst_id columns via add_edges_from_batch()
  • Auto-inferred or explicit vertex count

• build_bidirectional_index() — create both outgoing (CSR) and incoming (CSC) indices for undirected/reverse traversal

Why this matters

Currently lance-graph translates Cypher MATCH (a)-[:KNOWS]->(b) into SQL joins via DataFusion. For multi-hop queries, this means:

Operation	Current (SQL Joins)	With CSR Index
1-hop neighbor lookup	O(N) filter scan	O(1) offset + sequential read
k-hop traversal	O(N^k) self-joins	O(Σ degrees) pointer-chasing
Shortest path	Recursive CTEs	Direct BFS on CSR

Next steps (not in this PR)

1. Wire CSR into LanceNativePlanner to handle LogicalOperator::Expand
2. Persist CSR offset tables as Lance datasets alongside edge data
3. Incremental CSR updates on edge inserts (AL→CSR compaction, per BACH paper)
4. Combine graph traversal with Lance's vector search for hybrid queries

Test plan

• 22 unit tests covering: basic lookups, degree, empty graphs, isolated vertices, self-loops, parallel edges, RecordBatch construction, Arrow serialization roundtrip, BFS traversal (limited hops, disconnected, invalid start), shortest path (direct, multi-hop, same vertex, unreachable, invalid), bidirectional index, auto-inferred vertex count
• cargo clippy -p lance-graph -- -D warnings passes clean

Closes #159

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.81132% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/lance-graph/src/csr_index.rs	94.81%	22 Missing ⚠️

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