Add native KiCad test corpus

[Cimos]

Phase 0, PR 2 of the native KiCad plan. Test data and tooling only; no runtime crates change.

• Mad_RP2040 boards (corpus/native/kicad/mad_rp2040/{old,new}): Mad_RP2040.kicad_pcb at tags v0.0.0 and v0.0.1, MIT, saved by KiCad 9. All 13 files in each bundled demo pack are byte-identical, apart from dates, to the PCBWay pack in the matching Mad_RP2040 release, so these boards are the demo's true sources and the demo packs serve as their KiCad 9 reference export.
• KiCad 7 fixtures (fixtures/kicad7/): eight tiny boards, one behaviour each (track + arc, pad shapes, via spans, filled zone, unfilled zone, stroke text, renamed layer, bottom-side footprint). Reduced from the test_pads_inside_pads demo in Ubuntu's kicad-demos 7.0.11 (GPL-2+, test fixtures only, like corpus/pdf), and loaded and exported by kicad-cli 7.0.11. Reference exports are in oracles/.
• manifest.toml records path, hash, source, commit, licence, how each file was produced, header version, purpose, reference export and expected record counts. tools/test_manifest.py checks it; tools/export_oracle.sh regenerates reference exports.
• KiCad 6, 8 and 10 stay as explicit pending slots: no fixture claims a producer that did not write it.

All ten files parse with the etchy-kicad parser from #396 (the 4 MB board in about 94 ms).

CI wiring for the manifest checker is a one-line follow-up.

Refs #122