Expose od2net as a reusable library

[Robinlovelace]

Closes #97.

• move the existing pipeline from src/main.rs into src/lib.rs and re-export key modules/types so downstream bindings can call od2net programmatically
• keep src/main.rs as a tiny wrapper that parses CLI args (CliArgs) and calls run_pipeline, preserving the current command-line UX
• introduce OutputMetadata plus public APIs (Network, Request, CostFunction) needed by R/Python/WASM consumers and document the new entrypoints in the code comments

Testing

• cargo test

[Robinlovelace]

Disclaimer: AI generated.