Covers create_component_instance and _handle_add_schematic_component:
- default unit=1 behaviour
- explicit unit 2 and 4 written to (unit N) in schematic S-expression
- (instances ...) block also records the correct unit
- multiple units of the same reference placed independently
- handler param validation (missing schematicPath, missing component)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
