Boobytrap decoys die their own deaths

Stepping on a face-down token flips it: a blank leaves the board the
moment it is discovered, instead of all four indicators lingering
until the real one detonates. The trap itself still waits, the caster
still strolls over everything unharmed, and the chronicle narrates
the flip so the table learns which squares stopped being scary.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015RCWSTnb1KYTPyL4GmhGnF

packages/engine/test/expansion-terrain.test.ts

@@ -378,3 +378,49 @@ describe("a wave's force is spent as it travels", () => {
   });
 
 });
+
+describe("boobytrap decoys die their own deaths", () => {
+  it("a stepped-on blank token vanishes alone; the real one still waits", () => {
+    let { state } = newGame();
+    const me = activePlayer(state);
+    const enemy = state.players.find((p) => p.id !== me.id)!;
+    const view = boardView(state);
+    const open: Cell[] = [];
+    for (const key of Object.keys(view.cells)) {
+      const [x, y] = key.split(",").map(Number) as [number, number];
+      if (state.squareContents[key]) continue;
+      open.push({ x, y });
+      if (open.length === 4) break;
+    }
+    const bt = giveCard(state, me.id, "boobytrap");
+    state = must(state, me.id, {
+      type: "cast", instanceId: bt.instanceId, params: { cells: open },
+    });
+    state = must(state, me.id, { type: "endTurn", draw: 0 });
+    // The enemy steps onto a BLANK (open[1] — the real trap is open[0]).
+    const e = state.players.find((p) => p.id === enemy.id)!;
+    const blank = open[1]!;
+    for (const side of SIDES) {
+      const from = { x: blank.x + (side === "E" ? -1 : side === "W" ? 1 : 0),
+                     y: blank.y + (side === "S" ? -1 : side === "N" ? 1 : 0) };
+      if (!view.cells[cellKey(from)]) continue;
+      const t = stepTarget(view, from, side);
+      if (t.kind === "step" && cellKey(t.to) === cellKey(blank)) {
+        e.position = from;
+        const before = e.life;
+        const r = applyCommand(state, enemy.id, { type: "move", direction: side });
+        if (!r.ok) throw new Error(r.error);
+        state = r.state;
+        expect(r.events.some((ev) => ev.type === "boobytrapBlank")).toBe(true);
+        const after = state.players.find((p) => p.id === enemy.id)!;
+        expect(after.life).toBe(before);
+        const trap = state.boobytraps[0]!;
+        expect(trap.cells.length).toBe(3);
+        expect(trap.cells.some((c) => cellKey(c) === cellKey(blank))).toBe(false);
+        expect(trap.realKey).toBe(cellKey(open[0]!));
+        return;
+      }
+    }
+    throw new Error("setup: no approach to the blank token");
+  });
+});