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/simulation/interaction.rs

@@ -20,8 +20,12 @@ pub(crate) const MID_RANGE: u32 = 5;
 pub struct Interactable;
 
 /// Compute nearby interactions for the player character.
-/// For each visible entity in range, determines available verbs sorted by priority.
+/// For each entity in range, determines available verbs sorted by priority.
 /// Results are written to the NearbyInteractionBuffer for inclusion in ObserverSnapshot.
+///
+/// NOTE: Checks proximity only, not line-of-sight. The client filters
+/// interaction prompts against visible entities. Server-side LOS filtering
+/// is deferred until the interaction system can read the observer's visible set.
 #[allow(clippy::type_complexity)]
 pub fn compute_nearby_interactions(
     player_query: Query<(&TilePosition, &KnowledgeGraph), With<PlayerCharacter>>,
@@ -130,7 +134,10 @@ pub fn compute_nearby_interactions(
         let wire_id = registry
             .to_stable(entity)
             .map(|sid| sid.0)
-            .unwrap_or_else(|| entity.to_bits());
+            .unwrap_or_else(|| {
+                tracing::error!(?entity, "entity in interaction range but not in EntityRegistry");
+                entity.to_bits()
+            });
 
         buffer.interactions.push(NearbyInteraction {
             entity_id: wire_id,
@@ -148,6 +155,9 @@ pub fn compute_nearby_interactions(
 
 /// Buffer for nearby interaction results, consumed by snapshot generation.
 /// Field is private — use `take()` to drain results into the snapshot.
+///
+/// Global Resource — single-observer assumption (v0.1). D-009 multiplayer
+/// will refactor the entire observer + interaction pipeline to per-entity.
 #[derive(Resource, Debug, Default)]
 pub struct NearbyInteractionBuffer {
     interactions: Vec<NearbyInteraction>,