feat(simulation): add Zone Gate gauntlet room and zone crossing detection (#512)

New test_world room with two zones (Terminal/Corridor) separated by
a door. Adds ZoneCrossEventQueue resource and detect_zone_crossings
system to fire events when the player crosses zone boundaries.

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

server/src/simulation/zone.rs

@@ -4,6 +4,7 @@
 //!   - 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)
+//!   - Zone Gate Gauntlet room: crossing-event detection (QA #512)
 //!
 //! BTreeMap per D-010 principle 4 (deterministic iteration).
 
@@ -11,6 +12,8 @@ use std::collections::BTreeMap;
 
 use bevy_ecs::prelude::*;
 
+use crate::simulation::movement::{PlayerCharacter, TilePosition};
+
 /// Server-authoritative zone assignment for tiles.
 ///
 /// Each tile position maps to a zone ID. Tiles outside any defined zone
@@ -48,6 +51,70 @@ impl ZoneMap {
     }
 }
 
+/// Event emitted when the player crosses a zone boundary (D-077).
+///
+/// Queued once per tick when the player's tile zone_id differs from the
+/// previously recorded zone. `from` or `to` may be None for tiles outside
+/// any defined zone (corridors, unzoned transitions).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ZoneCrossEvent {
+    /// Zone the player left (None = no zone on previous tile).
+    pub from: Option<u16>,
+    /// Zone the player entered (None = no zone on new tile).
+    pub to: Option<u16>,
+}
+
+/// Resource: queue of zone-crossing events (drained each tick by consumers).
+#[derive(Resource, Default)]
+pub struct ZoneCrossEventQueue {
+    pub events: Vec<ZoneCrossEvent>,
+}
+
+impl ZoneCrossEventQueue {
+    pub fn push(&mut self, event: ZoneCrossEvent) {
+        self.events.push(event);
+    }
+
+    pub fn drain(&mut self) -> Vec<ZoneCrossEvent> {
+        std::mem::take(&mut self.events)
+    }
+}
+
+/// Tracks the player's zone from the previous tick.
+///
+/// Used by `detect_zone_crossings` to compare against the current tick's
+/// zone and emit a `ZoneCrossEvent` when they differ.
+#[derive(Resource, Debug, Default)]
+pub struct PreviousPlayerZone(pub Option<u16>);
+
+/// System: detect zone crossings and queue a `ZoneCrossEvent`.
+///
+/// Runs after `validate_movement` so the player position is current.
+/// Compares the player's current tile zone (from `ZoneMap`) against
+/// `PreviousPlayerZone`. Queues a `ZoneCrossEvent` on change and updates
+/// the resource for the next tick.
+pub fn detect_zone_crossings(
+    zone_map: Option<Res<ZoneMap>>,
+    mut previous_zone: ResMut<PreviousPlayerZone>,
+    player_query: Query<&TilePosition, With<PlayerCharacter>>,
+    mut queue: ResMut<ZoneCrossEventQueue>,
+) {
+    let Some(zone_map) = zone_map else {
+        return;
+    };
+    let Ok(pos) = player_query.single() else {
+        return;
+    };
+    let current_zone = zone_map.zone_at(pos.x, pos.y, pos.z);
+    if current_zone != previous_zone.0 {
+        queue.push(ZoneCrossEvent {
+            from: previous_zone.0,
+            to: current_zone,
+        });
+        previous_zone.0 = current_zone;
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;