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

server/src/simulation/zone.rs

@@ -0,0 +1,83 @@
+//! Zone map — spatial zone assignment for tiles (D-077, D-073).
+//!
+//! Maps tile coordinates to zone identifiers. Used by:
+//!   - Observer snapshot: enriches VisibleTile with zone_id
+//!   - Client AudioManager: zone crossfade triggers (D-073)
+//!   - Client fog shader: deep fog temperature tint (D-059 layer 3)
+//!
+//! BTreeMap per D-010 principle 4 (deterministic iteration).
+
+use std::collections::BTreeMap;
+
+use bevy_ecs::prelude::*;
+
+/// Server-authoritative zone assignment for tiles.
+///
+/// Each tile position maps to a zone ID. Tiles outside any defined zone
+/// (corridors, transition spaces) have no entry and return None.
+///
+/// Zone IDs are opaque u16 values — the client maintains its own
+/// `zone_id → zone_name / temperature_tint / ambient_layer` mapping.
+#[derive(Resource, Debug, Default)]
+pub struct ZoneMap {
+    zones: BTreeMap<(i32, i32, i32), u16>,
+}
+
+impl ZoneMap {
+    /// Look up the zone for a tile position.
+    pub fn zone_at(&self, x: i32, y: i32, z: i32) -> Option<u16> {
+        self.zones.get(&(x, y, z)).copied()
+    }
+
+    /// Assign a zone to a rectangular region of tiles.
+    /// Used by map builders to define zone boundaries.
+    pub fn set_rect(&mut self, ox: i32, oy: i32, w: i32, h: i32, z: i32, zone_id: u16) {
+        for y in oy..(oy + h) {
+            for x in ox..(ox + w) {
+                self.zones.insert((x, y, z), zone_id);
+            }
+        }
+    }
+
+    /// Assign a zone to a single tile.
+    pub fn set(&mut self, x: i32, y: i32, z: i32, zone_id: u16) {
+        self.zones.insert((x, y, z), zone_id);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn zone_at_returns_none_for_unset_tile() {
+        let map = ZoneMap::default();
+        assert_eq!(map.zone_at(10, 20, 0), None);
+    }
+
+    #[test]
+    fn set_rect_populates_zone() {
+        let mut map = ZoneMap::default();
+        map.set_rect(5, 10, 3, 2, 0, 42);
+
+        // Inside rect
+        assert_eq!(map.zone_at(5, 10, 0), Some(42));
+        assert_eq!(map.zone_at(7, 11, 0), Some(42));
+
+        // Outside rect
+        assert_eq!(map.zone_at(4, 10, 0), None);
+        assert_eq!(map.zone_at(8, 10, 0), None);
+        assert_eq!(map.zone_at(5, 12, 0), None);
+
+        // Wrong z-level
+        assert_eq!(map.zone_at(5, 10, 1), None);
+    }
+
+    #[test]
+    fn set_single_tile() {
+        let mut map = ZoneMap::default();
+        map.set(3, 7, 0, 99);
+        assert_eq!(map.zone_at(3, 7, 0), Some(99));
+        assert_eq!(map.zone_at(3, 8, 0), None);
+    }
+}