Add event emission for dropped GPS fixes in GTU7

[Copilot]

The GTU7 emit() method silently dropped GPS fixes when the output channel was full, making data loss invisible to applications.

Changes

• Event emission on drop: emit() now publishes EventInfo with metadata when fixes are dropped due to full channel
• Configurable buffer: Added GTU7Config.Buf to allow tuning channel size (default: 4)
• Lifecycle events: Added EventOpen and EventClose to Run() for observability
• Base integration: Embedded devices.Base in GTU7 struct to support event publishing

Pattern

Now follows VH400's approach for handling channel overflow:

func (g *GTU7) emit(f GPSFix) {
	select {
	case g.out <- f:
	default:
		g.Emit(devices.EventInfo, "gps fix dropped", nil, map[string]string{
			"reason": "output channel full",
		})
	}
}

Applications can monitor the event stream to detect slow consumer scenarios.

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[Copilot]

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