Finalize session: add FileSrc and librsadd tests

[kluff-dev]

This commit finalizes the session work by updating manifest metadata and adding substantial library code and tests.

• Update Cargo.toml (add/adjust package metadata and test/dev dependencies).
• Add/extend implementation in src/lib.rs: file-source helpers and librsadd-related functionality.
• Add unit/tests to exercise the new code paths and improve coverage.
• Include session identifier (92fe56f7-e472-425b-b4e9-e95dc5be664b) and related cleanup/comments.