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/bridge/types.rs

@@ -8,6 +8,15 @@ use serde::{Deserialize, Serialize};
 pub use crate::knowledge::types::{EntityVisibility, KnowledgeConfidence, RelationshipState};
 pub use crate::simulation::time::{DayPhase, TickRate};
 
+/// Wire protocol version for ObserverSnapshot.
+///
+/// Versioning strategy: flat struct + serde defaults for field evolution.
+/// Client and server are co-versioned (subprocess IPC per D-020), so protocol
+/// negotiation is unnecessary. Client should reject snapshots with version !=
+/// PROTOCOL_VERSION. New fields use #[serde(default)] only during the migration
+/// period, then the default is removed once both sides are updated.
+pub const PROTOCOL_VERSION: u8 = 4;
+
 /// The ONLY data structure crossing the client-server boundary (D-020)
 /// Contains all information visible to the observer at a given tick.
 ///