Add comprehensive documentation to GTU7 type

[Copilot]

The GTU7 type lacked documentation explaining what it is, how to use it, and key behavioral characteristics.

Changes

• Added godoc to GTU7 type explaining:
  • Driver purpose (parses NMEA sentences from GTU7 GPS module)
  • Complete usage example with context and channel consumption
  • RMC precedence over VTG for speed/course data
  • Channel buffering behavior (size 4, drops newest when full)
  • Lifecycle semantics (channel closed on context cancel/error)

Example

// GTU7 is a driver for the GTU7 GPS module that parses NMEA sentences
// (GGA, RMC, VTG) from a serial connection and emits GPSFix values.
//
// Basic usage:
//
//	cfg := GTU7Config{
//		Name: "gps",
//		Serial: drivers.SerialConfig{Port: "/dev/ttyUSB0", Baud: 9600},
//	}
//	gps := NewGTU7(cfg)
//	ctx := context.Background()
//	go gps.Run(ctx)
//	for fix := range gps.Out() {
//		// Process GPS fix
//	}
//
// Important behavioral notes:
//   - RMC sentences take precedence over VTG for speed and course data
//   - The Out channel is buffered (size 4); newest messages are dropped if the buffer is full
//   - Run will close the Out channel when the context is canceled or an error occurs
type GTU7 struct { ... }

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