Add documentation to GPSFix struct

[Copilot]

The exported GPSFix struct lacked documentation explaining field semantics, units, and valid values.

Changes:

• Added godoc for GPSFix documenting:
  • Coordinate units (decimal degrees, meters above MSL)
  • Speed field differences (SpeedKnots vs SpeedMPS)
  • Status field values (A=Active/valid, V=Void/invalid)
  • Date format (DDMMYY)
  • Quality indicator meanings (fix quality codes, HDOP, satellite count)

// GPSFix represents a GPS position fix aggregated from NMEA sentences
// (typically GPGGA, GPRMC, and GPVTG). Fields are populated as data becomes
// available from the sensor.
//
// Position coordinates:
//   - Lat, Lon: decimal degrees (negative for South/West)
//   - AltMeters: altitude in meters above mean sea level
//
// Quality indicators:
//   - Quality: GPS fix quality (0=invalid, 1=GPS fix, 2=DGPS fix, etc.)
//   - HDOP: horizontal dilution of precision
//   - Satellites: number of satellites in view
//
// Motion data:
//   - SpeedKnots: ground speed in knots
//   - SpeedMPS: ground speed in meters per second
//   - CourseDeg: course over ground in degrees (0-360, true north)
//
// Status fields:
//   - Status: RMC status ("A" = Active/valid, "V" = Void/invalid)
//   - Date: UTC date in DDMMYY format
type GPSFix struct { ... }

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[Copilot]

Pull request overview

This PR adds comprehensive godoc documentation to the exported GPSFix struct, which previously lacked documentation explaining its field semantics, units, and valid values.

Changes:

• Added detailed godoc comment block explaining the GPSFix struct's purpose, field groupings (position coordinates, quality indicators, motion data, status fields), units, and value meanings
• Removed inline comments for Status and Date fields as their explanations are now covered in the main struct documentation

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