Preserve library extra image directory

[haim1120]

Summary

• Preserve optional SPECCTRA library (extra_image_directory ...) descriptors when parsing DSN PCB library sections.
• Emit preserved extra image directory metadata from stringifyDsnJson so PCB parse -> stringify -> parse keeps external image lookup paths.
• Add focused regression coverage for (extra_image_directory "./extra-parts") inside library.

Verification

• bun test tests/dsn-pcb/library-extra-image-directory.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-extra-image-directory.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.