chore: scaffold tracks and tooling

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

js/package.json

@@ -0,0 +1,8 @@
+{
+  "name": "snake-course-js",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "test": "node --test \"test/*.test.js\""
+  }
+}

js/test.sh

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+#!/usr/bin/env bash
+# Usage: ./test.sh [MNN]   (no arg = all tests)
+set -euo pipefail
+cd "$(dirname "$0")"
+if [ $# -ge 1 ]; then
+  m=$(echo "$1" | tr 'A-Z' 'a-z')
+  exec node --test "test/${m}-*.test.js"
+else
+  exec node --test "test/*.test.js"
+fi

js/test/helpers.js

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+import { readFileSync } from 'node:fs'
+import { fileURLToPath } from 'node:url'
+import { dirname, join } from 'node:path'
+
+const here = dirname(fileURLToPath(import.meta.url))
+
+/** Load fixtures/<name>.json as { function, cases: [{ name, args, expected }] }. */
+export const loadFixture = (name) =>
+  JSON.parse(readFileSync(join(here, '..', '..', 'fixtures', `${name}.json`), 'utf8'))
+
+/** Deep-freeze for tests (independent of the student's deepFreeze). */
+export const freeze = (x) => {
+  if (x !== null && typeof x === 'object' && !Object.isFrozen(x)) {
+    Object.freeze(x)
+    Object.values(x).forEach(freeze)
+  }
+  return x
+}

js/test/m00-sanity.test.js

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+import test from 'node:test'
+import assert from 'node:assert/strict'
+import { loadFixture, freeze } from './helpers.js'
+
+test('m00 arithmetic', () => assert.equal(1 + 1, 2))
+
+test('m00 fixture loads and cases hold', () => {
+  const fx = loadFixture('m00-sanity')
+  assert.equal(fx.function, 'sanity')
+  fx.cases.forEach((c) => assert.equal(c.args[0] + c.args[1], c.expected, c.name))
+})
+
+test('m00 freeze is deep', () => {
+  const o = freeze({ a: { b: [1] } })
+  assert.ok(Object.isFrozen(o.a) && Object.isFrozen(o.a.b))
+})