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/sandbox.ts

@@ -0,0 +1,13 @@
+import type { OutboundCommandInput, SendResult } from './types';
+
+let seq = 0;
+
+/**
+ * Sandbox outbound sink — records a simulated send with no network I/O.
+ * (Real provider delivery is gated to P6+.)
+ */
+export const SandboxSink = {
+  async simulate(cmd: OutboundCommandInput): Promise<SendResult> {
+    return { providerRef: `sim-${cmd.channelType}-${++seq}`, outcome: 'SENT', latencyMs: 1 };
+  },
+};