Add synthetic schematic fixture corpus (KiCad / PDF / netlist / Altium stub)

[RFingAdam]

Goal

Phase 4b shipped schematic-aware analyzers, but the test suite cannot exercise the real parsing paths because no schematic fixtures exist under tests/fixtures/. We need synthetic, minimal-but-valid fixtures for each supported schematic format so parser tests can run hermetically (no copyrighted reference designs).

Acceptance criteria

• tests/fixtures/sample_2sheet.kicad_sch — hierarchical schematic with at least: 2 sheets, 4 components (R/C/U), explicit (wire) (label) (junction), in_bom / on_board properties, MPN property.
• tests/fixtures/sample_schematic.pdf — single-page PDF with extractable text containing refdes + values.
• tests/fixtures/sample_netlist.NET — ORCAD PSTXNET-style netlist with at least 3 nets.
• tests/fixtures/sample.SchDoc — bytes-only stub recognised by detect_format as Altium (full OLE parsing is a separate issue).
• tests/conftest.py exposes pytest fixtures for each.
• At least one test per fixture asserts the parser produces > 0 components and > 0 nets.

[RFingAdam]

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