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>

vitest.config.ts

@@ -10,6 +10,7 @@ export default defineConfig({
       // build step. Production builds still go through tsc/dist.
       '@insignia/iios-contracts': r('./packages/iios-contracts/src/index.ts'),
       '@insignia/iios-testkit': r('./packages/iios-testkit/src/index.ts'),
+      '@insignia/iios-adapter-sdk': r('./packages/iios-adapter-sdk/src/index.ts'),
     },
   },
   test: {