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/07-rule-resolution/reference/ts/src/engine/death.ts

@@ -5,7 +5,7 @@
 // ends. Then this step removes every marked minion at once.
 
 import type { Event } from "./event";
-import type { World } from "./world";
+import type { EntityId, World } from "./world";
 
 /**
  * Remove every mortally wounded or pending-destroy minion from play, simultaneously.
@@ -13,9 +13,17 @@ import type { World } from "./world";
  */
 export function deathCreationStep(world: World): Event[] {
   const dying = world.minionsInPlay().filter((id) => world.isDoomed(id));
-  // Every position is read before anything moves: removal is simultaneous, and each
-  // Death Event remembers where its minion stood when the step began.
-  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, position: world.boardIndex(id)! }));
-  for (const { id, position } of deaths) world.kill(id, position);
+  // 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).
+  const deaths = dying.map((id) => ({ type: "Destroyed" as const, id, before: survivorToTheRight(world, id) }));
+  for (const { id, before } of deaths) world.kill(id, before);
   return deaths;
 }
+
+/** The nearest minion to the right of `id` on its board that is not leaving play in this step, if any. */
+function survivorToTheRight(world: World, id: EntityId): EntityId | undefined {
+  const board = world.zone(world.ownerOf(id), "board");
+  return board.slice(board.indexOf(id) + 1).find((other) => !world.isDoomed(other));
+}