v0.3.0 - Gremlin Query Language Support

Major Features:
- Full ANTLR-based Gremlin parser (TinkerPop grammar)
- GremlinPlanBuilder maps Gremlin steps to ExecutionPlan operations
- Auto-detection: Queries starting with 'g.' parsed as Gremlin
- Same backend optimizations (pushdown, SQL generation) as Cypher

Supported Gremlin Steps:
- Source: V()
- Filter: hasLabel(), has()
- Traversal: out(), in(), both()
- Map: values(), valueMap()
- Aggregation: count(), sum(), mean(), min(), max()
- Modifier: limit(), order().by(), dedup(), as(), select()

Supported Predicates:
- Comparison: eq(), neq(), gt(), gte(), lt(), lte()
- Collection: within(), without()
- String: containing(), startingWith(), endingWith()

Breaking Changes:
- CypherEngine renamed to ProGraph (CypherEngine still works as alias)