#973: expand direct-Cypher varlen WHERE support beyond endpoint-only

[lmeyerov]

Summary

• Remove the compiler guard that rejected bounded/exact variable-length WHERE pattern predicates.
• Convert the old fail-fast test into positive execution coverage for exact/ranged/reverse and mixed predicate forms.
• Add bounded variable-length OR/XOR/NOT WHERE wrapper regressions.
• Refresh Cypher docs/changelog support-boundary language for Cypher multihop: expand direct variable-length support beyond endpoint-only patterns #973.

Validation

• python -m pytest -q graphistry/tests/compute/gfql/cypher/test_lowering.py -k "where_pattern and variable_length"
• python bin/ci_cypher_surface_guard.py
• uv run ruff check graphistry/compute/gfql/cypher/lowering.py graphistry/tests/compute/gfql/cypher/test_lowering.py

Closes #973.