Preserve library unit

[haim1120]

Summary

• Preserve optional SPECCTRA library-level (unit ...) descriptors when parsing DSN PCB library sections.
• Emit preserved library unit metadata from stringifyDsnJson so PCB parse -> stringify -> parse keeps section-specific library units.
• Add focused regression coverage for (unit mil) inside library.

Verification

• bun test tests/dsn-pcb/library-unit.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/library-unit.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.