fix(modules): a site runs one module; the installer refuses a second

A site is one game, and the module contract already has singletons that
assume it. registerTeamProvider holds one value per deployment, and a
second module registering one fails that module's whole load. The loader
scans alphabetically, so installing module-rust (which gains a Team
provider in its phase 9) beside module-uo would have taken uo down, not
rust.

install() now refuses, with 409 and before the artifact is downloaded,
any install whose id differs from a module already on the volume. An
upgrade of the installed module is still accepted; to change game,
remove the module first. Both install surfaces share this path, so a
MODULES declaration naming two modules installs the first and reports
the second as refused without failing the boot.

"Installed" means what the loader would scan: a directory named with a
module id that holds a module.json. An install's scratch directory and a
swap's aside copy do not count.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

server/test/moduleInstall.test.js

@@ -395,6 +395,79 @@ test('a failed upgrade leaves the previous version in place', async () => {
   assert.deepEqual(fs.readdirSync(tmpRoot), ['uo'])
 })
 
+// ── One module per site ────────────────────────────────────────────────────
+
+/** A second, different module's manifest and artifact, served beside the first. */
+function otherModuleRoutes(id = 'rust') {
+  const tarball = bundle({ id })
+  const artifact = `https://releases.example.com/mod/${id}-1.0.0.tar.gz`
+  const url = `https://releases.example.com/mod/${id}-1.0.0.json`
+  return {
+    url,
+    routes: {
+      [url]: manifestFor(tarball, { id, name: id, artifact: `${id}-1.0.0.tar.gz`, url: artifact }),
+      [artifact]: tarball,
+    },
+    artifact,
+  }
+}
+
+test('a second, different module is refused before anything is downloaded', async () => {
+  const first = goodRoutes()
+  await install.install({ url: MANIFEST_URL, hosts: HOSTS, fetchImpl: fakeFetch(first.routes) })
+
+  const other = otherModuleRoutes('rust')
+  const fetchImpl = fakeFetch(other.routes)
+  await assert.rejects(
+    () => install.install({ url: other.url, hosts: HOSTS, fetchImpl }),
+    (err) => {
+      assert.equal(err.name, 'InstallError')
+      // 409: nothing is wrong with the URL; the SITE is not in a state to take it.
+      assert.equal(err.status, 409)
+      assert.match(err.message, /already runs the module "uo"/)
+      assert.match(err.message, /remove it before installing "rust"/)
+      return true
+    },
+  )
+
+  // Refused on the manifest alone: the artifact was never fetched, and the
+  // volume holds exactly what it held before.
+  assert.ok(!fetchImpl.seen.includes(other.artifact), 'the artifact was not downloaded')
+  assert.deepEqual(fs.readdirSync(tmpRoot), ['uo'])
+})
+
+test('the same module is still an upgrade, and removing it frees the site for another', async () => {
+  const first = goodRoutes()
+  await install.install({ url: MANIFEST_URL, hosts: HOSTS, fetchImpl: fakeFetch(first.routes) })
+
+  // An upgrade of what is installed is exactly what the rule allows.
+  const second = goodRoutes({ version: '2.0.0', manifest: { version: '2.0.0' } })
+  second.routes[MANIFEST_URL] = manifestFor(second.tarball, { version: '2.0.0' })
+  const upgraded = await install.install({ url: MANIFEST_URL, hosts: HOSTS, fetchImpl: fakeFetch(second.routes) })
+  assert.equal(upgraded.replaced, true)
+
+  // And once it is gone, the site takes a different one.
+  await install.removeDir('uo')
+  const other = otherModuleRoutes('rust')
+  const result = await install.install({ url: other.url, hosts: HOSTS, fetchImpl: fakeFetch(other.routes) })
+  assert.equal(result.id, 'rust')
+  assert.deepEqual(install.installedIds(), ['rust'])
+})
+
+test('what counts as installed is what the loader would scan', () => {
+  // A real module, an install's scratch directory, a swap's aside copy and a
+  // directory with no module.json. Only the first is a module.
+  fs.mkdirSync(path.join(tmpRoot, 'uo'))
+  fs.writeFileSync(path.join(tmpRoot, 'uo', 'module.json'), '{}')
+  fs.mkdirSync(path.join(tmpRoot, '.install-rust-abc'))
+  fs.writeFileSync(path.join(tmpRoot, '.install-rust-abc', 'module.json'), '{}')
+  fs.mkdirSync(path.join(tmpRoot, 'uo.replaced-123'))
+  fs.writeFileSync(path.join(tmpRoot, 'uo.replaced-123', 'module.json'), '{}')
+  fs.mkdirSync(path.join(tmpRoot, 'notes'))
+
+  assert.deepEqual(install.installedIds(), ['uo'])
+})
+
 // ── The volume ─────────────────────────────────────────────────────────────
 
 test('moduleDir refuses an id that is not one', () => {