Root cause: kicad-sch-api doesn't back-populate comp.pins or sch.nets
on loaded schematics. All data is accessible through alternative APIs.
Pin extraction: use comp.get_symbol_definition().pins for metadata and
sch.list_component_pins(ref) for schematic-transformed positions.
Connectivity: new wire-walking engine using union-find on coordinates.
Walks wires, pin positions, labels, and power symbols to reconstruct
the net graph. Replaces broken ConnectivityAnalyzer/sch.nets fallbacks.
Eliminates 'unhashable type: Net' crash.
Hierarchy: use sch.sheets.get_sheet_hierarchy() instead of the broken
sheets.data.get("sheet", []) raw dict approach.
Labels: supplement sch.get_statistics() with sch.labels.get_statistics()
and sch.hierarchical_labels for accurate counts.
99 tests passing, lint clean.
