Cypher parser rejects boolean pattern existence expressions in RETURN #998

Benchmark-driven repro from pyg-bench on explicit master snapshot 7095102df:

Minimal repros that currently fail with [invalid-cypher-syntax] Invalid Cypher query syntax:

MATCH (person:Person {id: $personId})<-[:HAS_CREATOR]-(message:Message)<-[:LIKES]-(liker:Person)
RETURN liker.id AS personId, not((liker)-[:KNOWS]-(person)) AS isNew
LIMIT 5

MATCH (person:Person {id: $personId})<-[:HAS_CREATOR]-(message:Message)<-[:LIKES]-(liker:Person)
RETURN liker.id AS personId, not exists { (liker)-[:KNOWS]-(person) } AS isNew
LIMIT 5

Observed behavior:

• GFQLSyntaxError: [invalid-cypher-syntax] Invalid Cypher query syntax

Why this matters:

• This blocks LDBC SNB Interactive interactive-complex-7 / recent-likers before the later row-shaping issues can even be measured end to end.
• The benchmark query uses not((liker)-[:KNOWS]-(person)) AS isNew in the final RETURN.

Likely next blocker after this parser/front-end lane clears:

• #981 for multi-binding row/projection on the same query family.

Environment:

• benchmark repo: graphistry/pyg-bench
• runtime snapshot: pygraphistry master 7095102df
• engine: local pandas backend through graph.gfql(...)