Fix/v0.2.0 bugs testing and error path detection

[tiamilani]

Checklist

• Merged latest main into this branch
• Lint and tests pass locally
• New/changed code has tests

What does this PR do?
Fixes correctness and robustness issues, improves error reporting, ensures forward
compatibility, and adds the project's first automated test suite.

Parsing & calculations

• Validate coordinate systems and reject unsupported/out-of-range inputs.
• Make the 3 dB beam-border calculation more resilient.
• Guard beam-efficiency and data-loading against degenerate inputs.
• Require a schema when validation is requested.

Report generation

• Handle edge cases gracefully.
• Fix a circular import and route logging through a module logger.

Compatibility & maintenance

• Replace deprecated importlib.resources APIs.
• Internal refactor of the coordinate-transform dispatch and a formatting
  cleanup, with no change to public behavior.

Testing

• Add a pytest suite with fast, offline fixtures covering the above.

Notes for reviewers

• Stacked on the vendor-key normalization branch.
• Public API is unchanged but with better input validation and error detection.
• The new test suite isn't wired into CI in this PR; the existing lint, type,
  and build checks still run. Verified locally against Python 3.13.