feat(migrate): add @choojs/migrate — the choo-migrate v7→v8 codemod

Regex-based on purpose: converts simple top-level CJS patterns to ESM,
remaps package specifiers to their @choojs homes, points retired nano*
packages at built-in replacements (choo-lazy-route → lazy()), and reports
everything it refuses to guess at instead of guessing. Validated against
choo's own v7 example app: migrated output runs on v8 unmodified.

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

packages/migrate/test/transform.test.js

@@ -0,0 +1,92 @@
+import { test } from 'node:test'
+import assert from 'node:assert'
+
+import transform from '../lib/transform.js'
+
+test('converts a classic choo v7 header to v8 ESM', () => {
+  const src = [
+    "var choo = require('choo')",
+    "var html = require('choo/html')",
+    "var devtools = require('choo-devtools')",
+    '',
+    'var app = choo()',
+    "app.use(devtools())",
+    'module.exports = app'
+  ].join('\n')
+
+  const { code, changed, notes } = transform(src)
+  assert.ok(changed)
+  assert.match(code, /import choo from '@choojs\/core'/)
+  assert.match(code, /import html from '@choojs\/html'/)
+  assert.match(code, /import devtools from '@choojs\/devtools'/)
+  assert.match(code, /export default app/)
+  assert.deepStrictEqual(notes, [])
+})
+
+test('destructured and property requires', () => {
+  const src = [
+    "const { render } = require('some-lib')",
+    "const thing = require('other-lib').thing",
+    "const dflt = require('third-lib').default",
+    "require('./side-effect')"
+  ].join('\n')
+
+  const { code } = transform(src)
+  assert.match(code, /import { render } from 'some-lib'/)
+  assert.match(code, /import { thing as thing } from 'other-lib'/)
+  assert.match(code, /import dflt from 'third-lib'/)
+  assert.match(code, /import '\.\/side-effect'/)
+})
+
+test('already-ESM sources get specifiers remapped', () => {
+  const src = [
+    "import choo from 'choo'",
+    "import html from 'nanohtml'",
+    "import raw from 'nanohtml/raw'",
+    "import morph from 'nanomorph'",
+    "import Component from 'nanocomponent'"
+  ].join('\n')
+
+  const { code } = transform(src)
+  assert.match(code, /from '@choojs\/core'/)
+  assert.match(code, /from '@choojs\/html'\n/)
+  assert.match(code, /from '@choojs\/html\/raw'/)
+  assert.match(code, /from '@choojs\/html\/morph'/)
+  assert.match(code, /from '@choojs\/component'/)
+})
+
+test('retired packages produce notes, not rewrites', () => {
+  const src = [
+    "var nanobus = require('nanobus')",
+    "var lazyRoute = require('choo-lazy-route')"
+  ].join('\n')
+
+  const { code, notes } = transform(src)
+  assert.match(code, /from 'nanobus'/, 'retired specifier left for the human')
+  assert.ok(notes.some((n) => n.includes('nanobus')))
+  assert.ok(notes.some((n) => n.includes('lazy()')), 'choo-lazy-route points at lazy()')
+})
+
+test('dynamic import() specifiers are remapped too', () => {
+  const { code } = transform("const mod = await import('choo/html')")
+  assert.match(code, /import\('@choojs\/html'\)/)
+})
+
+test('unconvertible CJS is reported honestly', () => {
+  const src = [
+    "module.exports.helper = function () {}",
+    "function f () { const x = require('choo') }"
+  ].join('\n')
+
+  const { code, notes } = transform(src)
+  assert.match(code, /require\('@choojs\/core'\)/, 'specifier remapped even inside functions')
+  assert.ok(notes.some((n) => n.includes('module.exports')))
+  assert.ok(notes.some((n) => n.includes('require() calls remain')))
+})
+
+test('non-choo sources pass through untouched', () => {
+  const src = "import fs from 'node:fs'\nexport const x = 1\n"
+  const { changed, notes } = transform(src)
+  assert.strictEqual(changed, false)
+  assert.deepStrictEqual(notes, [])
+})