SB 4.1 Add cross-brain edge validation to GraphEngine

[carlgombert]

Task 4.1: Add cross-brain edge validation to GraphEngine

Wave: 7

Description

• Modify edge creation logic in crates/memento-graph-engine/src/processor.rs (or relevant store module) to verify source and target nodes belong to the active brain's database
• Return BrainError::CrossBrainReference if mismatch detected

Requirements

• 6.2, 6.3, 6.4