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/08-push-chains/reference/ts/src/engine/event.ts

@@ -14,9 +14,11 @@ export type Event =
   | { type: "SpellCast"; card: string }
   /**
    * A Death Event: the Death Creation Step moved `id` from play to the graveyard.
-   * `position` is the board index it had when the step began; its Deathrattle summons there.
+   * `before` is the nearest minion to its right that did not die in the same step, if any:
+   * its Deathrattle summons immediately left of that minion, or at the right end. It is a
+   * handle because a board index goes stale when a minion further left dies in the same step.
    */
-  | { type: "Destroyed"; id: EntityId; position: number }
+  | { type: "Destroyed"; id: EntityId; before: EntityId | undefined }
   | { type: "DeathPhase"; round: number }
   /** A minion was played from hand (it has just entered the board). Its Battlecry answers this. */
   | { type: "Played"; id: EntityId };