feat: EXISTS subquery support (simple, nested, with WHERE)

[DecisionNerd]

Problem

EXISTS subqueries (WHERE EXISTS { MATCH ... }) are not implemented. Queries using this syntax either fail to parse or execute incorrectly.

Remaining TCK Failures (as of Phase 9, 7 scenarios)

test_simple_subquery_without_where_clause
test_simple_subquery_without_where_clause_not_existing_pattern
test_simple_subquery_with_where_clause
test_simple_subquery_with_where_clause_not_existing_pattern
test_nested_simple_existential_subquery
test_nested_full_existential_subquery_with_pattern_predicate
test_full_existential_subquery_with_update_clause_should_fail

Failure Details

1. Simple EXISTS without WHERE (2): WHERE EXISTS { MATCH (n)-->(m) }
2. Simple EXISTS with WHERE (2): WHERE EXISTS { MATCH (n)-->(m) WHERE m.prop = 'x' }
3. Nested EXISTS (1): EXISTS inside EXISTS
4. EXISTS with pattern predicate (1): WHERE EXISTS { (n)-[:R]->(m) } combined with other predicates
5. EXISTS with update clause should fail (1): EXISTS { CREATE (n) } should raise SyntaxError at compile time

Layers Affected

All four: grammar (cypher.lark), AST (ast/), planner (planner.py), executor (executor.py / evaluator.py).

Acceptance Criteria

• WHERE EXISTS { MATCH (n)-->(m) } evaluates to true/false correctly
• WHERE EXISTS { MATCH (n)-->(m) WHERE m.active = true } filters by subquery condition
• Nested EXISTS { EXISTS { ... } } evaluates correctly
• EXISTS { CREATE (n) } raises SyntaxError at compile time