fix(simulation): determinism fixes — BTreeSet ordering, entity sort, mover sort

Replace HashSet with BTreeSet for visible_ids, sort visible_tiles by
coordinates, sort visible entities in snapshot by entity_id, and sort
movers by Entity bits in validate_movement. Required by D-010 principle 4
(deterministic simulation). Fixes #456, #457, #458.

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

server/src/perception/query.rs

@@ -5,7 +5,7 @@
 //! provide mode-specific FOV and visibility sector computation.
 //! v0.1 implements only NaturalVision.
 
-use std::collections::{HashMap, HashSet};
+use std::collections::{BTreeSet, HashMap};
 
 use bevy_ecs::prelude::*;
 
@@ -21,7 +21,7 @@ use crate::simulation::movement::{TilePosition, WalkabilityMap};
 #[derive(Resource, Default)]
 pub struct VisibilityGeometry {
     pub visible_tiles: Vec<VisibleTile>,
-    pub visible_positions: HashSet<(i32, i32)>,
+    pub visible_positions: BTreeSet<(i32, i32)>,
     pub sector_lookup: HashMap<(i32, i32), VisibilitySector>,
     pub observer_z: i32,
 }
@@ -64,7 +64,7 @@ impl PerceptionQuery for NaturalVision {
 
         let cone_tiles = apply_vision_cone(&fov, observer_pos.x, observer_pos.y, facing, &config);
 
-        let visible_tiles = cone_tiles
+        let mut visible_tiles: Vec<VisibleTile> = cone_tiles
             .iter()
             .map(|&(x, y, sector)| {
                 let tile_kind = if walkability.can_move_to(&TilePosition::new(x, y, z)) {
@@ -81,6 +81,7 @@ impl PerceptionQuery for NaturalVision {
                 }
             })
             .collect();
+        visible_tiles.sort_by_key(|t| (t.x, t.y));
 
         let visible_positions = cone_tiles.iter().map(|&(x, y, _)| (x, y)).collect();