Extract duplicated knots-to-MPS conversion constant

[Copilot]

The conversion factor 0.514444 (knots to meters per second) was duplicated in two NMEA parsing functions, violating DRY and obscuring intent.

Changes:

• Extracted knotsToMPS = 0.514444 as a package-level constant in sensors/gtu7.go
• Updated parseGPRMC and parseGPVTG to use the named constant

Before:

fix.SpeedMPS = v * 0.514444  // Repeated in two functions

After:

const knotsToMPS = 0.514444

// In parseGPRMC and parseGPVTG:
fix.SpeedMPS = v * knotsToMPS

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