fix(weather): harden diagnostic edge cases

[stacknil]

Summary

• Harden edge-case handling in the weather diagnostics toolkit.
• Add explicit validation for ensemble member values, missing regional reductions, non-finite model inputs, empty metric inputs, and mismatched advection fields.
• Add ruff to the scoped weather-toolkit CI workflow.
• Document the missing-data and ensemble-spread behavior.

Validation

• python -m ruff check .
• python -m pytest -> 16 passed
• python -m compileall src scripts
• CLI help smoke checks for run_thermodynamic_check.py, run_dynamics_summary.py, and run_synthetic_ensemble.py

Privacy and Data Boundary

• Candidate-file scan returned 0 restricted artifact files and 0 text privacy matches.
• No raw weather datasets, course documents, compressed archives, binary generated artifacts, or local-machine materials are included.