test(chess/primitives): lock primitive registry count at 22 (T28)

Adds a tiny assertion test that imports the barrel and checks
PRIMITIVE_REGISTRY.list().length === 22 — 15 pre-existing primitives
plus the 7 trigger primitives added in Wave 2 (on-move, on-turn-end,
on-promotion, on-check-received, on-check-delivered,
on-moved-onto-square, on-captured).

This is the registry-count portion of T28. The dnd-kit dependency
install already landed in commit 26e708b; bundle-size measurement was
performed out-of-band (273kb gz total for the chess demo including
Vite + React + Rete engine + dnd-kit + all components).

packages/chess/src/modifiers/primitives/registry-count.test.ts

@@ -0,0 +1,18 @@
+import { describe, it, expect } from "vitest";
+import { PRIMITIVE_REGISTRY } from "./index.js";
+
+describe("PRIMITIVE_REGISTRY", () => {
+  it("should have exactly 22 registered primitives after barrel import", () => {
+    const count = PRIMITIVE_REGISTRY.list().length;
+    expect(count).toBe(22);
+  });
+
+  it("should list all primitive kinds with non-empty descriptor objects", () => {
+    const primitives = PRIMITIVE_REGISTRY.list();
+    expect(primitives.length).toBeGreaterThan(0);
+    primitives.forEach((p) => {
+      expect(p).toHaveProperty("kind");
+      expect(p.kind).toBeTruthy();
+    });
+  });
+});