fix(reference): Death Events remember a right-hand neighbour, not a board index

A Deathrattle summon used the dying minion's board index from before the
Death Creation Step removed anything. That index is stale when a minion to
its left dies in the same step, so the token landed one place too far right.

Each Death Event now records `before`: the nearest minion to the right that
is not dying in the same step. The position handed to the effect context is
resolved when the trigger resolves: the current index of `before`, or the
right end. Applied to the Rust and TS references of Modules 3-10.

Module 3 gains two tests, documented in scenarios.md:
test_03_summon_position_after_simultaneous_deaths (fails before the fix) and
test_03_simultaneous_summons_keep_board_order.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

course/05-assemble/reference/rust/src/engine/death.rs

@@ -5,26 +5,37 @@
 //! ends. Then this step removes every marked minion at once.
 
 use super::event::Event;
-use super::world::World;
+use super::world::{EntityId, World, Zone};
 
 /// Remove every mortally wounded or pending-destroy minion from play, simultaneously.
 /// Returns their Death Events in order of play; empty if nothing died.
 pub fn death_creation_step(world: &mut World) -> Vec<Event> {
-    // Every position is read before anything moves: removal is simultaneous, and each
-    // Death Event remembers where its minion stood when the step began.
+    // Every neighbour is read before anything moves: removal is simultaneous. Each Death
+    // Event remembers where its minion stood as a handle, the minion that stays to its
+    // right, not as a board index: an index goes stale as soon as a minion further left
+    // leaves play in the same step (Module 1: hold handles, not indexes).
     let mut deaths = Vec::new();
     for id in world.minions_in_play() {
         if world.is_doomed(id) {
-            let position = world
-                .board_index(id)
-                .expect("a minion in play is on the board");
-            deaths.push(Event::Destroyed { id, position });
+            let before = survivor_to_the_right(world, id);
+            deaths.push(Event::Destroyed { id, before });
         }
     }
     for death in &deaths {
-        if let Event::Destroyed { id, position } = *death {
-            world.kill(id, position);
+        if let Event::Destroyed { id, before } = *death {
+            world.kill(id, before);
         }
     }
     deaths
 }
+
+/// The nearest minion to the right of `id` on its board that is not leaving play in this step, if any.
+fn survivor_to_the_right(world: &World, id: EntityId) -> Option<EntityId> {
+    world
+        .zone(world.owner_of(id), Zone::Board)
+        .iter()
+        .copied()
+        .skip_while(|&other| other != id)
+        .skip(1)
+        .find(|&other| !world.is_doomed(other))
+}