Add KML export for Google Earth visualization

[Copilot]

Implements KML export functionality to visualize vessel tracks in Google Earth, addressing a roadmap item.

Changes

• KMLExporter class - Exports single or multiple AIS tracks to valid KML 2.2 format with vessel metadata (position, name, MMSI, callsign, speed, course, dimensions)
• 19 unit tests - Covers null handling, empty collections, KML structure validation, and metadata inclusion
• Demo application - Shows export workflow from NMEA messages to KML file
• Documentation - README updated with usage examples and roadmap status

Usage

AISTracker tracker = new AISTracker();
tracker.update(aisInputStream);

try (FileWriter writer = new FileWriter("vessels.kml")) {
    KMLExporter.exportToKML(tracker.getAisTracks(), writer);
}

Defensive coding handles missing vessel data (dimensions, position) gracefully. Module exports added for new dk.tbsalling.ais.exporter package.

Original prompt

Suggest new feature

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