feat(iios): EmailAdapter (email-native normalize + threading) in adapter-sdk

Adds a canonical EmailInboundPayload + EmailAdapter implementing the
ChannelAdapter contract: HMAC signature verify + normalize with email-native
threading (whole reply chain → one IIOS thread via references/inReplyTo root),
messageId as providerEventId for dedup, html→text fallback, Re:/Fwd: stripping.
Plus emailInboundFixture/emailReplyFixture and unit tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/iios-adapter-sdk/src/fixtures.ts

@@ -1,7 +1,8 @@
 import { hmacSign } from './hmac';
 import type { WebhookPayload } from './webhook-adapter';
+import type { EmailInboundPayload } from './email-adapter';
 
-/** Email-shaped provider payload (as the adapter's normalize input). */
+/** Email-shaped WebhookPayload (legacy generic-webhook fixture). */
 export const emailFixture: WebhookPayload = {
   eventId: 'email-1',
   from: 'someone@example.com',
@@ -11,6 +12,30 @@ export const emailFixture: WebhookPayload = {
   text: 'Hello, I have a question.',
 };
 
+/** A first inbound email (starts a thread) for the EmailAdapter. */
+export const emailInboundFixture: EmailInboundPayload = {
+  messageId: '<m1@example.com>',
+  from: 'buyer@example.com',
+  fromName: 'Buyer',
+  to: ['support@tower.example'],
+  subject: 'Need help with my order',
+  text: 'Hi, my order has not arrived yet.',
+  occurredAt: '2026-07-03T10:00:00.000Z',
+};
+
+/** A reply to `emailInboundFixture` — must land on the SAME email thread. */
+export const emailReplyFixture: EmailInboundPayload = {
+  messageId: '<m2@example.com>',
+  from: 'buyer@example.com',
+  fromName: 'Buyer',
+  to: ['support@tower.example'],
+  subject: 'Re: Need help with my order',
+  text: 'Any update on this?',
+  inReplyTo: '<m1@example.com>',
+  references: ['<m1@example.com>'],
+  occurredAt: '2026-07-03T11:00:00.000Z',
+};
+
 /** WhatsApp-shaped provider payload. */
 export const whatsappFixture: WebhookPayload = {
   eventId: 'wa-1',