init: rough companion app stub

lib/domain/models/particle.dart

@@ -0,0 +1,17 @@
+/// Single particle in a particle filter cloud snapshot.
+class Particle {
+  const Particle({required this.x, required this.y, required this.weight});
+
+  /// Normalised 0..1 coordinates (same space as [Position]).
+  final double x;
+  final double y;
+
+  /// Unnormalised likelihood weight.
+  final double weight;
+
+  factory Particle.fromJson(Map<String, dynamic> json) => Particle(
+        x: (json['x'] as num).toDouble(),
+        y: (json['y'] as num).toDouble(),
+        weight: (json['weight'] as num).toDouble(),
+      );
+}