Editor: Phase 4 - State Reconciliation

[neumaennl]

Summary:
Implement the StateReconciler logic for incremental diagram updates, diff/merge algorithm, and animation. (ADR 001)

Related ADR:
docs/architecture/001-editor-transition.md — Phase 4

Acceptance criteria:

• Diff algorithm exists and passes snapshot tests
• Incremental re-render for changed nodes
• Animation/transition for visual feedback
• Large-schema performance tested

Tasks:

• Implement schema diff algorithm
• Incremental update paths for diagram renderer
• Visual transition for add/remove/update
• Performance tests for large schemas