Preserve session logical part descriptors

[haim1120]

Summary

• Preserve optional SPECCTRA session placement (logical_part ...) descriptors when parsing placement records.
• Emit preserved logical part descriptors from stringifyDsnSession so session parse -> stringify -> parse keeps logical-part metadata.
• Add focused regression coverage for two placements with distinct logical part ids.

Verification

• bun test tests/dsn-pcb/session-logical-part.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-session.ts tests/dsn-pcb/session-logical-part.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.