Fix CPR zone ambiguity causing aircraft to appear 100km away from actual position

PROBLEM:
- Aircraft were appearing ~100km from receiver when actually ~5km away
- CPR (Compact Position Reporting) algorithm has zone ambiguity issue
- Without reference position, aircraft can appear in wrong 6-degree zones

SOLUTION:
- Add receiver reference position to CPR decoder for zone resolution
- Modified NewDecoder() to accept reference latitude/longitude parameters
- Implement distance-based zone selection (choose solution closest to receiver)
- Updated all decoder instantiations to pass receiver coordinates

TECHNICAL CHANGES:
- decoder.go: Add refLatitude/refLongitude fields and zone ambiguity resolution
- beast.go: Pass source coordinates to NewDecoder()
- beast-dump/main.go: Use default coordinates (0,0) for command-line tool
- merger.go: Add position update debugging for verification
- JavaScript: Add coordinate validation and update logging

RESULT:
- Aircraft now appear at correct distances from receiver
- CPR zone selection based on proximity to known receiver location
- Resolves fundamental ADS-B position accuracy issue

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

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

cmd/beast-dump/main.go

@@ -100,7 +100,7 @@ func parseFlags() *Config {
 func NewBeastDumper(config *Config) *BeastDumper {
 	return &BeastDumper{
 		config:  config,
-		decoder: modes.NewDecoder(),
+		decoder: modes.NewDecoder(0.0, 0.0), // beast-dump doesn't have reference position, use default
 		stats: struct {
 			totalMessages    int64
 			validMessages    int64