Rewrite KiCadSchematicParser using sexpdata (proper S-expr tree walker)

[RFingAdam]

Problem

parsers/schematic_parser.py::KiCadSchematicParser is regex-based today. It misses hierarchical sheets, pin-net mapping via (wire)/(label)/(junction), the in_bom/on_board DNP flags, and Datasheet/MPN properties. Downstream schematic analyzers (power_topology, protection_circuits, decoupling_per_ic) only operate in their degraded "no pin-net mapping" mode against KiCad inputs.

Acceptance criteria

• Use sexpdata (already in the dependency tree or add it) for proper S-expression tree traversal.
• Hierarchical sheets recursed; sheet path recorded per component.
• Net resolution from (wire) + (label) + (junction) produces ParsedComponent.pins with {pin_number, net} entries.
• in_bom / on_board properties drive a dnp flag on ParsedComponent.
• Datasheet, MPN, Manufacturer properties exposed via ParsedComponent.part_number / manufacturer.
• Round-trip test using the synthetic sample_2sheet.kicad_sch fixture asserts component count, net count, pin-net mapping, DNP flag, and hierarchical sheet path.
• All existing tests still pass.

[RFingAdam]

Closed by PR #125 (commit ab37f5a). All CI checks (lint / typecheck / test 3.10-3.12) green.