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/ts/src/engine/world.ts

@@ -275,10 +275,10 @@ export class World {
   }
 
   /** Move a minion from play to its owner's graveyard, recording its Death Event. */
-  kill(id: EntityId, position: number): void {
+  kill(id: EntityId, before: EntityId | undefined): void {
     this.entity(id).pendingDestroy = false;
     this.moveTo(id, "graveyard");
-    this.record({ type: "Destroyed", id, position });
+    this.record({ type: "Destroyed", id, before });
   }
 
   gainArmor(player: PlayerId, amount: number): void {