Add unit tests for persistence layer

[RBhupi]

Context
The persistence layer has minimal test coverage. Failure paths, schema evolution, and cross-module contract validation are untested.

Persistence layer

• Write + read round-trip returns identical data
• Parquet schema evolution — new column in append does not corrupt existing rows (NaN fill)
• Duplicate insert raises, does not silently overwrite
• Missing file path raises immediately on read
• Catalog path stored relative to radar_dir, not base_dir
• Registry correctly records and retrieves radar location

More Contract boundaries test

• Passing a wrong type at a contract boundary raises immediately
• Missing required field in contract dataclass raises on construction
• Each module's run() output satisfies the downstream module's input contract
• No module imports directly from another module (static import check)

Constraints

• Temporary in-memory or tmp_path SQLite — no shared test databases
• Synthetic DataFrames only — no real Parquet fixtures
• Pure unit tests — no pipeline execution, no real data
• Import boundary test can use ast or importlib to inspect statically

Priority
Low