fix: Type flatten as yielding items instead of pages

The declared yield type of SeamPaginator.flatten was the page array
while the generator yields single items, so iterating flatten typed
each item as an array of the resource. The mistyping also hid a broken
README example that read a property off the wrong variable.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B8xeJm2Hd923k8uo6eoFd2

Author: claude

README.md

@@ -350,7 +350,7 @@ const pages = seam.createPaginator(
 )
 
 for await (const device of pages.flatten()) {
-  console.log(devices.name)
+  console.log(device.display_name)
 }
 ```
 

src/lib/seam-paginator.ts

@@ -142,9 +142,7 @@ export class SeamPaginator<
   /**
    * Yields each item across all pages, fetching the next page as needed.
    */
-  async *flatten(): AsyncGenerator<
-    EnsureReadonlyArray<TResponse[TResponseKey]>
-  > {
+  async *flatten(): AsyncGenerator<ElementOfArray<TResponse[TResponseKey]>> {
     let [current, pagination] = await this.firstPage()
     for (const item of current) {
       yield item
@@ -174,6 +172,8 @@ export class SeamPaginator<
 
 type EnsureReadonlyArray<T> = T extends readonly any[] ? T : never
 type EnsureMutableArray<T> = T extends any[] ? T : never
+type ElementOfArray<T> =
+  T extends ReadonlyArray<infer TElement> ? TElement : never
 
 interface PaginationData {
   has_next_page: boolean

test/seam/connect/seam-paginator.test.ts

@@ -1,7 +1,7 @@
 import test from 'ava'
 import { getTestServer } from 'fixtures/seam/connect/api.js'
 
-import { SeamHttp, SeamPaginator } from '@seamapi/http/connect'
+import { type Device, SeamHttp, SeamPaginator } from '@seamapi/http/connect'
 
 test('SeamPaginator: creates a SeamPaginator', async (t) => {
   const { seed, endpoint } = await getTestServer(t)
@@ -80,14 +80,21 @@ test('SeamPaginator: flatten allows iteration over all devices', async (t) => {
   const allDevices = await seam.devices.list()
   const pages = seam.createPaginator(seam.devices.list({ limit: 1 }))
 
-  const devices = []
+  const deviceIds = []
   for await (const device of pages.flatten()) {
-    devices.push(device)
+    expectType<Device>(device)
+
+    // @ts-expect-error Verify flatten yields single items, not pages.
+    expectType<Device[]>(device)
+
+    deviceIds.push(device.device_id)
   }
-  t.true(devices.length > 1)
-  t.is(devices.length, allDevices.length)
+  t.true(deviceIds.length > 1)
+  t.is(deviceIds.length, allDevices.length)
 })
 
+const expectType = <Expected>(_value: Expected): void => {}
+
 test('SeamPaginator: instance allows iteration over all pages', async (t) => {
   const { seed, endpoint } = await getTestServer(t)
   const seam = SeamHttp.fromApiKey(seed.seam_apikey1_token, { endpoint })