feat(simulation): add knowledge graph system per D-041 (#361, #362, #363, #365)

Implement per-entity KnowledgeGraph component with BTreeMap storage
for deterministic iteration (D-010). StableEntityId + EntityRegistry
for entity identity across save/load. KnowledgeEventQueue + processing
system for decoupled knowledge updates. Decay system runs once per
game-minute. All types conform to D-041 canonical structs.

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

server/src/knowledge/mod.rs

@@ -0,0 +1,42 @@
+//! Knowledge graph module (D-041).
+//!
+//! Implements information boundaries (D-010 principle 2): every piece of
+//! state is tagged with who knows it. Per-entity KnowledgeGraph component
+//! tracks what each entity knows about others and the world.
+
+use bevy_app::prelude::*;
+use bevy_ecs::prelude::*;
+
+pub mod events;
+pub mod graph;
+pub mod registry;
+pub mod types;
+
+pub use events::{
+    KnowledgeEvent, KnowledgeEventQueue, KnowledgeEventType,
+};
+pub use graph::KnowledgeGraph;
+pub use registry::{EntityRegistry, StableEntityId};
+pub use types::*;
+
+/// Knowledge system plugin.
+/// Registers resources and systems for knowledge graph processing.
+pub struct KnowledgePlugin;
+
+impl Plugin for KnowledgePlugin {
+    fn build(&self, app: &mut App) {
+        app.init_resource::<KnowledgeEventQueue>()
+            .init_resource::<EntityRegistry>()
+            .init_resource::<DecayThresholds>()
+            .add_systems(
+                Update,
+                (
+                    events::process_knowledge_events
+                        .after(crate::perception::observer::compute_observer_snapshot),
+                    events::decay_knowledge
+                        .after(events::process_knowledge_events),
+                ),
+            );
+        tracing::debug!("KnowledgePlugin initialized");
+    }
+}