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>

internal/client/beast.go

@@ -72,7 +72,7 @@ func NewBeastClient(source *merger.Source, merger *merger.Merger) *BeastClient {
 	return &BeastClient{
 		source:         source,
 		merger:         merger,
-		decoder:        modes.NewDecoder(),
+		decoder:        modes.NewDecoder(source.Latitude, source.Longitude),
 		msgChan:        make(chan *beast.Message, 1000),
 		errChan:        make(chan error, 10),
 		stopChan:       make(chan struct{}),