Remove arbitrary 600kt speed cap from Mode S decoder

The decoder was capping all speeds at 600kt before validation, which caused:
1. Corrupt helicopter data to show false 600kt readings
2. Legitimate high-speed aircraft to be incorrectly limited
3. Validation logic in merger to miss corrupt data detection

The merger already has proper speed validation at 2000kt (Mach 3) and
position-based validation that catches impossible speeds from corrupt data
regardless of aircraft type.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/modes/decoder.go

@@ -621,10 +621,6 @@ func (d *Decoder) decodeVelocity(data []byte, aircraft *Aircraft) {
 		// Calculate ground speed in knots (rounded to integer)
 		speedKnots := math.Sqrt(ewVel*ewVel + nsVel*nsVel)
 
-		// Validate speed range (0-600 knots for civilian aircraft)
-		if speedKnots > 600 {
-			speedKnots = 600 // Cap at reasonable maximum
-		}
 		aircraft.GroundSpeed = int(math.Round(speedKnots))
 
 		// Calculate track in degrees (0-359)