Add VRS JSON format support for readsb integration

Added comprehensive support for VRS (Virtual Radar Server) JSON format
as a simpler alternative to Beast binary protocol, enabling integration
with readsb --net-vrs-port output.

## Key Features:
- **VRS JSON Parser**: Stream parsing of newline-delimited JSON aircraft data
- **VRS Client**: TCP client with automatic reconnection and error recovery
- **Mixed Format Support**: Use Beast and VRS sources simultaneously
- **Enhanced Aircraft Data**: Added VRS-specific fields (registration, type, operator)
- **Position Source Tracking**: Identifies ADS-B, MLAT, TIS-B, and satellite positions

## Implementation:
- `internal/vrs/parser.go`: VRS JSON message parsing and validation
- `internal/client/vrs.go`: VRS TCP client implementation
- Enhanced `MultiSourceClient` to support both Beast and VRS formats
- Extended `Aircraft` struct with validity flags and additional metadata
- Updated configuration to include `format` field ("beast" or "vrs")

## Testing:
- Successfully tested against svovel:33005 VRS JSON stream
- Verified aircraft data parsing and position tracking
- Confirmed mixed-format operation with existing Beast clients

## Documentation:
- Updated README.md with VRS format configuration examples
- Enhanced ARCHITECTURE.md with VRS parser documentation
- Added data format comparison and configuration guide

This enables simpler integration with modern readsb installations while
maintaining full backward compatibility with existing Beast deployments.

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

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

internal/modes/decoder.go

@@ -155,6 +155,26 @@ type Aircraft struct {
 	SelectedAltitude int     // MCP/FCU selected altitude in feet
 	SelectedHeading  float64 // MCP/FCU selected heading in degrees
 	BaroSetting      float64 // Barometric pressure setting (QNH) in millibars
+
+	// Additional fields from VRS JSON and extended sources
+	Registration      string // Aircraft registration (e.g., "N12345")
+	AircraftType      string // Aircraft type (e.g., "B738")
+	Operator          string // Airline or operator name
+	
+	// Validity flags for optional fields (used by VRS and other sources)
+	CallsignValid     bool // Whether callsign is valid
+	PositionValid     bool // Whether position is valid
+	AltitudeValid     bool // Whether altitude is valid
+	GeomAltitudeValid bool // Whether geometric altitude is valid
+	GroundSpeedValid  bool // Whether ground speed is valid
+	TrackValid        bool // Whether track is valid
+	VerticalRateValid bool // Whether vertical rate is valid
+	SquawkValid       bool // Whether squawk code is valid
+	OnGroundValid     bool // Whether on-ground status is valid
+
+	// Position source indicators
+	PositionMLAT bool // Position derived from MLAT
+	PositionTISB bool // Position from TIS-B
 }
 
 // Decoder handles Mode S and ADS-B message decoding with CPR position resolution.