feat: pattern predicates in WHERE clause (openCypher 9)

[dylanbstorey]

Summary

• Pattern predicates: Bare relationship patterns in WHERE clauses now work as implicit existence checks per openCypher 9 spec. WHERE NOT (n)-[:REL]->() is syntactic sugar for WHERE NOT EXISTS((n)-[:REL]->()).
• Direction fix: Fixed pre-existing bug where EXISTS(pattern) always assumed outgoing direction — incoming (<-) and undirected (-) patterns now generate correct SQL.
• Bumps version to 0.3.10.

Closes GQLITE-T-0138.

Changes

File	Change
cypher_gram.y	Two new expr productions for 3- and 5-element pattern predicates; %expect 4→9 (GLR-safe)
transform_expr_predicate.c	Direction-aware join generation for EXISTS subqueries
14_pattern_predicates.sql	17 functional tests: all directions, typed/untyped, NOT/AND/OR/XOR, EXISTS equivalence

Test plan

• 5300 unit test assertions pass (0 failures)
• All existing functional tests pass
• 17 new functional tests for pattern predicates pass
• Original reported query MATCH (n {entity_type: 'Note'}) WHERE NOT (n)-[:BELONGS_TO]->() ... now works