feat(simulation): wire direct observation to knowledge graph (#364, #367)

Perception system emits DirectObservation/LeftLOS knowledge events
based on snapshot visibility. Entities entering LOS get Direct
confidence; leaving LOS downgrades to KnowsDetails. System params
are optional for backward compatibility when KnowledgePlugin is not
registered. 28 knowledge graph unit tests included.

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

server/src/bridge/mod.rs

@@ -177,6 +177,8 @@ impl Plugin for BridgePlugin {
                     crate::perception::observer::compute_observer_snapshot
                         .after(crate::simulation::movement::validate_movement)
                         .before(crate::simulation::time::advance_tick),
+                    crate::perception::observation::emit_observation_events
+                        .after(crate::perception::observer::compute_observer_snapshot),
                     send_bridge_snapshot
                         .after(crate::perception::observer::compute_observer_snapshot),
                 ),