feat(devtools): add @choojs/devtools — window.choo console tooling

Carries forward the choo-devtools 3.x essentials: live state handle,
emit-from-console, event log, nanotiming measures via PerformanceObserver,
state copy(), help(). No-op on the server; verbose logging opt-in via
localStorage.CHOO_DEVTOOLS_VERBOSE.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014NgfSjHE11oFpoSnLVKLXd

packages/devtools/test/devtools.test.js

@@ -0,0 +1,51 @@
+import { test, before } from 'node:test'
+import assert from 'node:assert'
+import { Window } from 'happy-dom'
+
+let choo, html, devtools
+
+before(async () => {
+  const win = new Window({ url: 'http://localhost/' })
+  globalThis.window = win
+  globalThis.document = win.document
+  globalThis.requestAnimationFrame = win.requestAnimationFrame.bind(win)
+  ;({ default: choo } = await import('@choojs/core'))
+  ;({ default: html } = await import('@choojs/html/browser'))
+  ;({ default: devtools } = await import('../index.js'))
+})
+
+test('devtools exposes window.choo with live state, log and emit', async () => {
+  const app = choo()
+  app.use(devtools())
+  app.use((state, emitter) => {
+    state.count = 0
+    emitter.on('bump', (n) => { state.count += n })
+  })
+  app.route('/', (state) => html`<div>${state.count}</div>`)
+  app.start()
+
+  assert.ok(window.choo, 'window.choo exists')
+  assert.strictEqual(window.choo.state.count, 0, 'live state exposed')
+
+  window.choo.emit('bump', 2)
+  assert.strictEqual(window.choo.state.count, 2, 'emit from the console handle works')
+  assert.ok(window.choo.log.some((e) => e.name === 'bump'), 'events land in the log')
+  assert.strictEqual(typeof window.choo.help, 'function')
+
+  const json = JSON.parse(window.choo.copy())
+  assert.strictEqual(json.count, 2, 'copy() serializes state')
+  assert.strictEqual(json.cache, null, 'state.cache not serialized')
+})
+
+test('devtools is a no-op on the server', async () => {
+  // fresh import in a windowless world is covered by the guard: calling
+  // the store with no window must not throw or touch globals
+  const store = devtools()
+  const hadWindow = globalThis.window
+  delete globalThis.window
+  try {
+    assert.doesNotThrow(() => store({}, { on () {} }, {}))
+  } finally {
+    globalThis.window = hadWindow
+  }
+})