feat: render particle clouds for selected tags

lib/domain/models/particle.dart

@@ -12,6 +12,32 @@ class Particle {
   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(),
+        weight: (json['w'] as num).toDouble(),
       );
 }
+
+/// Position estimate from the particle filter (weighted mean).
+class Estimate {
+  const Estimate({required this.x, required this.y});
+
+  final double x;
+  final double y;
+
+  factory Estimate.fromJson(Map<String, dynamic> json) => Estimate(
+        x: (json['x'] as num).toDouble(),
+        y: (json['y'] as num).toDouble(),
+      );
+}
+
+/// A single `particles_updated` push from the server: estimate + raw cloud.
+class ParticleSnapshot {
+  const ParticleSnapshot({
+    required this.tagId,
+    required this.estimate,
+    required this.particles,
+  });
+
+  final String tagId;
+  final Estimate estimate;
+  final List<Particle> particles;
+}