feat(simulation): #523 add zone_id to ObserverSnapshot (D-077, protocol v11)

Server-tracked zone_id on VisibleTile for D-073 zone crossfade and
D-059 deep fog temperature tint. ZoneMap resource backed by BTreeMap,
observer enrichment in snapshot assembly. Backwards-compatible: v10
payloads deserialize with zone_id: None.

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

server/src/simulation/mod.rs

@@ -19,6 +19,7 @@ pub mod sound;
 pub mod stance;
 pub mod tier;
 pub mod time;
+pub mod zone;
 
 /// Core simulation plugin
 /// Manages simulation time, RNG, input processing, and tier transitions