Preserve padstack via sites

[haim1120]

Summary

• Preserve DSN padstack (via_site ...) descriptors while parsing PCB library padstacks.
• Emit preserved via_site descriptors from stringifyDsnJson, including coordinate sites and off markers.
• Add focused round-trip regression coverage for padstack via_site metadata.

Verification

• bun test tests/dsn-pcb/padstack-via-sites.test.ts
• bun test
• bunx tsc --noEmit
• bunx biome check lib/dsn-pcb/types.ts lib/dsn-pcb/dsn-json-to-circuit-json/parse-dsn-to-dsn-json.ts lib/dsn-pcb/circuit-json-to-dsn-json/stringify-dsn-json.ts tests/dsn-pcb/padstack-via-sites.test.ts
• bun run build
• git diff --check

/claim #54

Transparency note: AI-assisted with Codex; I reviewed the diff and verified it locally before submission.