Preserve placement place control

[haim1120]

Summary

• Preserve optional SPECCTRA placement (place_control (flip_style ...)) descriptors when parsing placement sections.
• Emit preserved place_control flip_style metadata from both DSN PCB and session stringifiers.
• Add focused regression coverage proving session parse -> stringify -> parse keeps rotate_first flip_style metadata.

Verification

• bun test tests/dsn-pcb/placement-place-control.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 lib/dsn-pcb/circuit-json-to-dsn-json/stringify-dsn-session.ts tests/dsn-pcb/placement-place-control.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.