feat(gfql): add same-path WHERE executor

[lmeyerov]

Summary

• add same-path WHERE executor (Yannakakis-style semijoin reduction)
• extend chain/GFQL plumbing for WHERE clauses and execution modes
• add reference + compute tests for WHERE semantics

Stack

• Depends on: perf(hop): traversal optimizations and fast path #911
• Related (parallel): chore(bench): add GFQL benchmarks and CI job #913 (benchmarks + CI job)

Testing

• uv run pytest graphistry/tests/compute/test_chain_where.py graphistry/tests/compute/test_chain.py graphistry/tests/compute/predicates/test_str.py
• uv run pytest tests/gfql/ref