feat: HMAC-SHA256 signing module with native tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

firmware/lib/hmac/hmac.h

@@ -0,0 +1,16 @@
+// firmware/src/hmac.h
+#pragma once
+#include <stdint.h>
+
+#ifdef NATIVE_TEST
+#include <string>
+using HString = std::string;
+#else
+#include <Arduino.h>
+using HString = String;
+#endif
+
+// Returns lowercase hex-encoded HMAC-SHA256 signature.
+// Message signed: device_id + ":" + timestamp_str + ":" + hex(sha256(body))
+HString hmac_sign(const HString& secret_hex, const HString& device_id,
+                  uint32_t timestamp, const HString& body);