v0.4.0 - Mutation support milestone

Features:
- Complete SPARQL 1.1 Update support (INSERT/DELETE DATA, MODIFY, CLEAR/DROP GRAPH)
- Cypher mutations (CREATE, SET, DELETE, DETACH DELETE, MERGE, REMOVE)
- Mutable Pandas backend with MutableDataFrameMixin
- Mutable Spark backend with Delta Lake support
- ID generation strategies (uuid, sequence)