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/bridge/text_renderer.rs

@@ -268,6 +268,7 @@ mod tests {
                 z: 0,
                 visibility: VisibilitySector::Forward,
                 tile_kind: TileKind::Floor,
+                zone_id: None,
             }],
             nearby_interactions: vec![NearbyInteraction {
                 entity_id: 100,

server/src/bridge/types.rs

@@ -15,7 +15,7 @@ pub use crate::simulation::time::{DayPhase, TickRate};
 /// 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 = 10;
+pub const PROTOCOL_VERSION: u8 = 11;
 
 /// The ONLY data structure crossing the client-server boundary (D-020)
 /// Contains all information visible to the observer at a given tick.
@@ -30,10 +30,11 @@ pub const PROTOCOL_VERSION: u8 = 10;
 /// v9 adds: blocked_entities (#514, debug field for LOS-blocked entities).
 /// v10 adds: sound_events (#124, D-038 server sound event pipeline),
 ///           rng_seed (#527, deterministic replay — completes WRONG button loop).
+/// v11 adds: zone_id on VisibleTile (#523, D-077 OQ-09 resolution + D-073 crossfade).
 /// Future fields: ambient sound events, HUD state (D-020 expansion).
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ObserverSnapshot {
-    /// Protocol version for forward compatibility. Current: 10.
+    /// Protocol version for forward compatibility. Current: 11.
     pub version: u8,
     /// Simulation tick when this snapshot was produced
     pub tick: u64,
@@ -209,6 +210,13 @@ pub struct VisibleTile {
     /// Tile type for client rendering (floor, wall, door, object)
     #[serde(default)]
     pub tile_kind: TileKind,
+    /// Zone identifier for this tile (D-077 OQ-09, D-073 crossfade).
+    /// Server-authoritative zone assignment. Client maps zone_id to:
+    ///   - Audio crossfade target (D-073)
+    ///   - Deep fog temperature tint (D-059 layer 3)
+    /// None for tiles outside any defined zone (corridors, transition spaces).
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub zone_id: Option<u16>,
 }
 
 /// Tile type for rendering. Derived from WalkabilityMap on the server side.