Create data/signals.json with 20 seed signals (FM broadcast, airband,
ISM bands, maritime VHF, AIS, ACARS, ADS-B, POCSAG, cellular, WiFi/BT,
amateur radio, DAB, PMR446, FRS/GMRS, NOAA weather radio). Point
frequencies adjusted to ±500 Hz windows; fixed-value bandwidth_ranges
widened to strict min < max windows required by schema.
Add tests/test_signals_json.py with 9 schema validation tests covering
id uniqueness, required string fields, frequency range validity,
bandwidth range, modulation casing, categories, region codes, and
sigidwiki URLs.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
James Smith