gfql/cypher: anti-semi-join lowering for WHERE NOT pattern (#1031 slice 2 phase 2b)

[lmeyerov]

Parent: #1031

This issue tracks #1031 slice 2 phase 2b: execute WHERE NOT pattern via anti-semi-join lowering/runtime path.

Context

• AST plumbing landed in feat(gfql/cypher): NOT-pattern AST plumbing (#1031 slice 2 phase 2a) #1233 (WherePatternPredicate.negated)
• Current lowering still raises unsupported for negated pattern

Scope

• Replace lowering unsupported hooks for negated pattern with executable lowering
• Add runtime anti-semi filtering path
• Add tests for seed and mixed row+NOT shapes

Reference docs

• plans/1031-slices-2-3-4/HANDOFF.md
• plans/1031-slices-2-3-4/findings/slice-2-phase-2b-engine.md

[lmeyerov]

Implementation PR opened: #1238

• Branch: m4-1031-slice-2-phase-2b
• Scope: executes WHERE NOT (pattern) via anti-semi row pre-filter lowering/runtime
• Validation: full cypher lowering suite + touched-module mypy

Next on merge: #1237 (bound-aliases + IC10) and #1236 (OR/XOR-around-pattern).

[lmeyerov]

Status update (2026-04-29):\n\n- CI red root cause identified and addressed: tck-gfql strict-improvement drift on expr-pattern1-19 (expected old validation error, now succeeds).\n- Contract patch pushed in tck-gfql branch m4-1031-slice-2-phase-2b, commit fb57977.\n- Rerun triggered for failed checks on PR #1238 (run 25098679675), with tck-gfql rerun currently in progress.\n- Review flow completed for PR #1238: 0 BLOCKER, 0 IMPORTANT, 1 optional suggestion (add explicit connected OPTIONAL MATCH + NOT-pattern runtime test).\n\nReview artifacts: plans/1031-slices-2-3-4/review-PR-1238/