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/03-death-step/reference/rust/src/engine/world.rs

@@ -276,10 +276,10 @@ impl World {
     }
 
     /// Move a minion from play to its owner's graveyard, recording its Death Event.
-    pub fn kill(&mut self, id: EntityId, position: usize) {
+    pub fn kill(&mut self, id: EntityId, before: Option<EntityId>) {
         self.entity_mut(id).pending_destroy = false;
         self.move_to(id, Zone::Graveyard);
-        self.record(Event::Destroyed { id, position });
+        self.record(Event::Destroyed { id, before });
     }
 
     pub fn gain_armor(&mut self, player: PlayerId, amount: i32) {