feat: Module 11 becomes 13; diff-tracks fuzzer; align the two reference tracks

- Rename course/11-break-your-engine to 13-break-your-engine and update every
  link and mention; start-module explains that Module 13 has no tests to start.
- tools/diff-tracks.mjs (npm run diff:tracks): stages both references of a
  module, generates a seeded random SpellGrid level and script per seed, pipes
  it to the Rust and TypeScript CLIs and diffs their output; crashes on one or
  both sides fail. stagePlay takes a module number.
- register_assembled: the TypeScript track compared only the text of an
  existing Assembled card, so a base re-registered with a new cost (or a
  different split) silently rewrote the earlier card. It now compares every
  field the engine reads, as Rust does (Modules 5-10).
  New test_05_assemble_never_rewrites_an_earlier_card.
- SpellGrid CLIs: coordinates and chooser numbers are 1-9 plain digits in both
  tracks (Rust accepted signs, TypeScript accepted "+2", "0x2", "1e0"); Rust
  quotes card names verbatim instead of with Debug escapes; Rust joins the
  minion list with "; " instead of rewriting ", ". Two new CLI tests per track.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/test/start-module.test.mjs

@@ -98,3 +98,15 @@ test("start_module_00_not_startable", () => {
   assert.notEqual(r.code, 0);
   assert.match(r.out, /naive-engine/);
 });
+
+test("start_module_explains_a_module_with_no_tests", () => {
+  // A module made only of reading and exercises (Module 13): no tests for either track.
+  const course = copyFixture("course");
+  mkdirSync(join(course, "06-no-tests"));
+  writeFileSync(join(course, "06-no-tests", "lesson.md"), "# Module 6\n");
+  const ws = copyFixture("ws-start");
+  const r = run("start-module.mjs", ["06", ws], { COURSE_ROOT: course });
+  assert.equal(r.code, 1);
+  assert.match(r.out, /Module 06 has no tests to start/);
+  assert.doesNotMatch(r.out, /finish module/);
+});