fix(server): add error logging, protocol version, and architecture docs

- Replace silent Entity::to_bits() fallbacks with tracing::error in
  observer.rs and interaction.rs (makes unregistered entities loud)
- Add PROTOCOL_VERSION constant to types.rs, use in observer snapshot
- Document single-observer assumption on NearbyInteractionBuffer
- Document proximity-only (no LOS) limitation on compute_nearby_interactions

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

server/src/perception/observer.rs

@@ -118,7 +118,10 @@ pub fn compute_observer_snapshot(
         let wire_id = registry
             .to_stable(entity)
             .map(|sid| sid.0)
-            .unwrap_or_else(|| entity.to_bits());
+            .unwrap_or_else(|| {
+                tracing::error!(?entity, "entity visible but not in EntityRegistry");
+                entity.to_bits()
+            });
         visible_entity_bits.insert(wire_id);
         entities.push(VisibleEntity {
             entity_id: wire_id,
@@ -160,7 +163,7 @@ pub fn compute_observer_snapshot(
 
     // Step 8: Assemble snapshot (v4: added nearby_interactions)
     buffer.snapshot = Some(ObserverSnapshot {
-        version: 4,
+        version: crate::bridge::types::PROTOCOL_VERSION,
         tick: time.tick,
         game_time,
         player_facing: facing,