Fix bottom silkscreen layer for tracks and arcs

[StealthEyeLLC]

Fixes #398.

Summary

• Use the parsed EasyEDA layer when converting silkscreen TRACK shapes.
• Use the parsed EasyEDA layer when converting silkscreen ARC shapes.
• Add a regression test covering injected layer 4 bottom-side TRACK and ARC shapes.

Layer 4 is already recognized as bottom silkscreen by getSideFromLayer, and CIRCLE silkscreen conversion already uses that helper. This applies the same mapping to TRACK and ARC instead of hardcoding top.

Tests

bun test tests/convert-to-soup-tests/silkscreen-bottom-layer.test.ts
bun test tests/parse-tests/single-letter-shape-schema.test.ts
bun run build
bun test tests/convert-to-ts/C472489-to-ts.test.ts

I also ran the full bun test suite. It hit the existing 5000ms timeout on tests/convert-to-ts/C472489-to-ts.test.ts during the full run, then that same test passed in isolation in about 5.46s.