🧪 test(nmea): add test file and comprehensive tests for rmc.py

[google-labs-jules]

🎯 What: Missing test file for src/nmea/rmc.py addressed by creating a dedicated tests/test_nmea/test_rmc.py.
📊 Coverage: Added comprehensive tests for NMEA RMC sentence regex extraction (all match groups) and the lonlat processing method, covering north/east, south/west, exact zeros, and edge cases where zero-padding strings lead to empty conditions.
✨ Result: Test coverage significantly improved for the rmc.py file, establishing a safe baseline for this module. Also cleaned up test_rmc_zda_znt.py into test_zda_znt.py.

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PR created automatically by Jules for task 4834563210980612454 started by @schwehr

[google-labs-jules]

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