Fix RMC precedence flags not resetting when data unavailable

[Copilot]

The haveRMCSpeed and haveRMCCourse flags were set once when RMC provided data but never cleared, preventing VTG sentences from updating speed/course when subsequent RMC sentences lacked those fields.

Changes

• Reset precedence flags to false when RMC sentence fields are empty/NaN
• Added test case verifying VTG fallback when RMC stops providing data

Implementation

if !math.IsNaN(fix.SpeedKnots) {
    last.SpeedKnots = fix.SpeedKnots
    last.SpeedMPS = fix.SpeedMPS
    haveRMCSpeed = true
} else {
    haveRMCSpeed = false  // Now resets per-sentence
}

This implements per-sentence precedence rather than sticky global state.

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