fix(simulation): address PR #10 review — plugin wiring, stale reset, warnings

- Register KnowledgePlugin in main.rs and game_loop test (Tyre critical)
- Add KnowledgeGraph component to player spawn (Tyre critical)
- Reset Stale -> Active on fresh direct observation (Hoshe warning)
- Make registry/queue non-optional in emit_observation_events (Tyre/Hoshe)
- Add tracing::warn for missing EntityRegistry entries (Hoshe warning)
- Replace HashSet with Vec for small entity ID lookups (Hoshe suggestion)
- Add const static assertion for KnowledgeConfidence ordering (Hoshe)
- Add is_empty() and known_facts_iter() to KnowledgeGraph (Tyre)
- Add decay_skips_non_minute_ticks and observe_resets_stale tests (Hoshe)

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

server/src/knowledge/graph.rs

@@ -95,6 +95,16 @@ impl KnowledgeGraph {
         self.facts.len()
     }
 
+    /// Whether the knowledge graph has no entries at all.
+    pub fn is_empty(&self) -> bool {
+        self.entities.is_empty() && self.facts.is_empty()
+    }
+
+    /// Iterate all known facts (deterministic order via BTreeMap).
+    pub fn known_facts_iter(&self) -> impl Iterator<Item = (&FactId, &FactKnowledge)> {
+        self.facts.iter()
+    }
+
     // --- Write Operations ---
 
     /// Record a direct observation of another entity (entity is in LOS).
@@ -119,8 +129,12 @@ impl KnowledgeGraph {
         entry.last_updated_tick = tick;
         entry.confidence = KnowledgeConfidence::Direct;
         entry.source = KnowledgeSource::DirectObservation { tick };
-        // Do NOT reset state here -- a Contradicted entry stays Contradicted
-        // even if you're looking at the entity right now.
+        // Stale entries become Active again on fresh observation.
+        // Contradicted entries stay Contradicted even if you're looking
+        // at the entity right now — the contradiction is still unresolved.
+        if entry.state == KnowledgeState::Stale {
+            entry.state = KnowledgeState::Active;
+        }
     }
 
     /// Entity has left the observer's LOS. Downgrade from Direct.
@@ -355,6 +369,21 @@ mod tests {
         assert_eq!(entry.last_known_position, Some(make_position(6, 10)));
     }
 
+    #[test]
+    fn observe_resets_stale_to_active() {
+        let mut g = KnowledgeGraph::new();
+        let target = StableId(1);
+        g.observe_entity(target, make_position(5, 10), 100);
+        // Mark as stale (would be done by decay system)
+        g.entities.get_mut(&target).unwrap().state = KnowledgeState::Stale;
+
+        // Re-observe — stale should reset to Active
+        g.observe_entity(target, make_position(6, 10), 200);
+        let entry = g.entity_knowledge(&target).unwrap();
+        assert_eq!(entry.state, KnowledgeState::Active);
+        assert_eq!(entry.confidence, KnowledgeConfidence::Direct);
+    }
+
     #[test]
     fn with_background_creates_facts() {
         let facts = vec![