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/rust/src/engine/effect.rs

@@ -82,7 +82,7 @@ fn targets(target: Target, ctx: &Ctx) -> Vec<EntityId> {
 }
 
 /// Where a summon goes: to the right of a source still on the board; where a dead
-/// source stood (its Death Event's position); at the right end for a spell.
+/// source stood (left of the minion its Death Event remembers); at the right end for a spell.
 fn summon_position(ctx: &Ctx) -> Option<usize> {
     match ctx.source.and_then(|s| ctx.world.board_index(s)) {
         Some(index) => Some(index + 1),