Make GTU7 output channel buffer size configurable

[Copilot]

The GTU7 sensor hardcoded its output channel buffer to 4, which is insufficient for GPS modules outputting at 1-10 Hz when consumers are slow.

Changes

• Added Buf int field to GTU7Config with default of 16
• Updated NewGTU7 to use configurable buffer size
• Added tests verifying default and custom buffer behavior

Follows the existing pattern from VH400Config:

type GTU7Config struct {
    Name    string
    Serial  drivers.SerialConfig
    Factory drivers.SerialFactory
    
    // Buf sizes the out channel. Default 16.
    Buf int
    
    // Test injection
    Reader io.Reader
}

Usage:

// Use default buffer of 16
gps := NewGTU7(GTU7Config{Serial: cfg})

// Or customize for high-rate GPS
gps := NewGTU7(GTU7Config{Serial: cfg, Buf: 32})

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[Copilot]

Pull request overview

This PR makes the GTU7 GPS sensor output channel buffer size configurable, replacing the hardcoded 4-element buffer with a configurable Buf field that defaults to 16. This addresses GPS data loss issues when modules output at 1-10 Hz with slow consumers.

Changes:

• Added configurable Buf field to GTU7Config with default value of 16
• Removed unused dependencies (maciej/bme280, nfnt/resize, golang.org/x/image) that are only referenced in archived code
• Added comprehensive tests for buffer size configuration

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
sensors/gtu7.go	Added Buf field to config and logic to default to 16 when value is ≤ 0
sensors/gtu7_test.go	Added comprehensive tests for default, custom, zero, and negative buffer sizes
go.mod	Removed unused direct dependencies and promoted golang.org/x/sys from indirect to direct
go.sum	Removed checksums for unused dependencies

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