feat(cypher): support connected MATCH + OPTIONAL MATCH (#996) - #1021

Summary

• feat(cypher): Support MATCH (connected-path) OPTIONAL MATCH ... RETURN queries where the non-optional MATCH is a connected path. Unblocks LDBC SNB Interactive interactive-short-7 (IS7) and any query combining a connected traversal with an optional extension and mixed-alias or CASE-expression projections (Cypher/GFQL cannot lower MATCH + OPTIONAL MATCH reply-author flag queries after a connected MATCH #996).
• fix(tests): Replace cudf.DataFrame.from_pandas() → cudf.from_pandas() across the GFQL Cypher test suite for RAPIDS 25.02 + 26.02 compatibility (42 call sites).

Architecture

New compilation path _compile_connected_optional_match() in lowering.py:

1. Detects the 2-clause pattern (connected non-optional + optional) before _merged_match_clause() runs
2. Lowers each clause independently via lower_match_clause()
3. Builds a post_join_chain using the same _build_projection_plan + return_() + _lower_order_by_clause infrastructure as the normal path

New execution path _apply_connected_optional_match() in gfql_unified.py:

1. Runs each chain with rows(binding_ops=...) to produce binding-row tables
2. Left-outer-joins on shared node alias columns
3. Delegates RETURN / ORDER BY / SKIP / LIMIT to the standard row pipeline via _chain_dispatch

No hand-rolled expression evaluation — full feature parity with the normal Cypher path because the same row pipeline handles all expression types.

Test plan

• 19 new connected OPTIONAL MATCH tests covering:
  • Expression breadth: type(), coalesce(), arithmetic, CASE WHEN ... IS NULL
  • Join edge cases: no matches, all match, multi-row per base, empty base, two shared aliases, integer IDs, custom node column name, longer optional chains (2-hop)
  • Post-projection: ORDER BY DESC, SKIP+LIMIT, DISTINCT
• Full test_lowering.py CPU: 546 passed, 49 skipped
• Full GFQL GPU on DGX (RAPIDS 26.02): 886 passed, 5 skipped (igraph only), 0 failed
• Broader suites (test_gfql, test_compute_chain, test_chain_let, test_parser): 286 passed
• typecheck: success (204 files)
• lint: all checks passed

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