Embed devices.Base in GTU7 to implement complete Device interface

[Copilot]

The GTU7 struct was missing the Events() method required by the Device interface. VH400 and other sensors implement this by embedding devices.Base.

Changes

• Embed devices.Base in GTU7 struct to provide Name() and Events() methods
• Initialize Base in NewGTU7 with event buffer size of 16
• Emit lifecycle events in Run(): EventOpen at start, EventClose/EventError on exit, and close events channel

Example

// GTU7 now fully implements Device interface
var _ devices.Device = (*sensors.GTU7)(nil)
var _ devices.Source[sensors.GPSFix] = (*sensors.GTU7)(nil)

gps := NewGTU7(GTU7Config{...})
events := gps.Events()  // ✓ now available

go func() {
    for e := range events {
        log.Printf("%s: %s", e.Kind, e.Msg)
    }
}()

Pattern now matches VH400 and polling-based sensors.

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