feat(sdk): P5.1 @insignia/iios-adapter-sdk — ChannelAdapter contract + HMAC webhook adapter + sandbox + fixtures

Contract (capability incl. requiresFollowupFetch seam, verifySignature, normalize,
fetch?, send), HMAC-SHA256 sign/verify (timing-safe), reference WebhookAdapter
(normalize→IngestInteractionRequest), SandboxSink (no network), email/whatsapp
fixtures. CJS build (consumed by the NestJS service). 4 tests; boundary updated.

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

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

@@ -0,0 +1,5 @@
+export * from './types';
+export { hmacSign, hmacVerify } from './hmac';
+export { SandboxSink } from './sandbox';
+export { WebhookAdapter, type WebhookPayload } from './webhook-adapter';
+export { emailFixture, whatsappFixture, signedFixture } from './fixtures';