fix: derive feels-like when NWS observation omits it (#721)

[Orinks]

Summary

• derive wind chill and heat index for NWS current conditions when the observation omits explicit feels-like fields
• feed the derived values through the existing thermal-comfort sanitizer so the main view and tray placeholder both get coherent feels-like data
• add parser regressions for missing wind chill and missing heat index observations

Testing

• .venv/bin/pytest -q tests/test_nws_current_parser.py tests/test_thermal_comfort_sanity.py tests/test_current_conditions.py tests/test_system_tray.py tests/test_weather_client_fusion.py
• .venv/bin/ruff check src/accessiweather/thermal_comfort.py src/accessiweather/weather_client_nws_parsers.py tests/test_nws_current_parser.py

Closes #721