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/package.json

@@ -0,0 +1,15 @@
+{
+  "name": "@insignia/iios-adapter-sdk",
+  "version": "0.0.0",
+  "private": true,
+  "main": "dist/index.js",
+  "types": "dist/index.d.ts",
+  "files": ["dist"],
+  "scripts": {
+    "build": "tsc -p tsconfig.json",
+    "typecheck": "tsc -p tsconfig.json --noEmit"
+  },
+  "dependencies": {
+    "@insignia/iios-contracts": "workspace:*"
+  }
+}