feat(perception): add observation event generator

Implements ticket #239 for Sprint 3:
- ObservationTrigger enum: RoutineDeviation (NPC not at expected
  routine location), Absence (expected location visible but NPC
  missing), NewEntity (unknown entity in LOS)
- ObservationEventQueue resource with push/drain/len/is_empty
- generate_observation_events system runs after emit_observation_events
  but before process_knowledge_events so it can compare current
  snapshot against previous-tick knowledge state
- Populate PerceptionPlugin with ObservationEventQueue resource and
  system registration with correct ordering constraints

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/perception/mod.rs

@@ -3,7 +3,9 @@
 // Generates ObserverSnapshot for client rendering
 
 use bevy_app::prelude::*;
+use bevy_ecs::schedule::IntoScheduleConfigs;
 
+pub mod interpretation;
 pub mod observation;
 pub mod observer;
 pub mod shadowcast;
@@ -14,8 +16,14 @@ pub mod vision_cone;
 pub struct PerceptionPlugin;
 
 impl Plugin for PerceptionPlugin {
-    fn build(&self, _app: &mut App) {
-        // Stub implementation - will be populated in phase 2
+    fn build(&self, app: &mut App) {
+        app.init_resource::<interpretation::ObservationEventQueue>()
+            .add_systems(
+                Update,
+                interpretation::generate_observation_events
+                    .after(observation::emit_observation_events)
+                    .before(crate::knowledge::events::process_knowledge_events),
+            );
         tracing::debug!("PerceptionPlugin initialized");
     }
 }