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/10-spellgrid/reference/ts/src/engine/queue.ts

@@ -75,8 +75,8 @@ export class Ctx {
     /** The chosen target, if any. */
     readonly target?: EntityId,
     /**
-     * The source's board position. For a Deathrattle, the source has left play,
-     * and this is the position its Death Event remembered.
+     * The source's board position. For a Deathrattle, the source has left play, and
+     * this is where it stood: immediately left of the minion its Death Event remembers.
      */
     readonly position?: number,
   ) {}
@@ -219,12 +219,26 @@ function react(world: World, events: Event[]): void {
     const times = activations(world, world.ownerOf(source), trigger.on);
     for (let i = 0; i < times; i++) {
       world.record({ type: "Triggered", source, on: trigger.on });
-      const position = event.type === "Destroyed" && event.id === source ? event.position : world.boardIndex(source);
+      const position =
+        event.type === "Destroyed" && event.id === source
+          ? positionLeftOf(world, world.ownerOf(source), event.before)
+          : world.boardIndex(source);
       runEffects(world, trigger.effects, new Ctx(world, world.ownerOf(source), source, undefined, position));
     }
   }
 }
 
+/**
+ * The board index, right now, immediately left of `before` on `owner`'s board; the
+ * right end if `before` is undefined or no longer there. A Death Event remembers the
+ * minion, not an index, so this still holds after other minions have left or arrived.
+ */
+function positionLeftOf(world: World, owner: PlayerId, before: EntityId | undefined): number {
+  const board = world.zone(owner, "board");
+  const index = before === undefined ? -1 : board.indexOf(before);
+  return index < 0 ? board.length : index;
+}
+
 /**
  * Who may respond to `event`, in order of play: every minion in play and, for a
  * Death Event, the minion that died. It has left play, but its Deathrattle still answers.