feat: add unit parameter to add_schematic_component for multi-unit symbols

Allows placing a specific unit (A=1, B=2, C=3, …) of a multi-unit KiCad
symbol rather than always defaulting to unit 1. Required for quad
optocouplers, dual op-amps, and other multi-unit parts where each channel
must be placed independently.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

python/kicad_interface.py

@@ -717,6 +717,7 @@ class KiCADInterface:
             footprint = component.get("footprint", "")
             x = component.get("x", 0)
             y = component.get("y", 0)
+            unit = component.get("unit", 1)
 
             # Derive project path from schematic path for project-local library resolution
             schematic_file = Path(schematic_path)
@@ -732,6 +733,7 @@ class KiCADInterface:
                 footprint=footprint,
                 x=x,
                 y=y,
+                unit=unit,
                 project_path=derived_project_path,
             )