Import ESP_UniClock (upstream 1189810)

Vendored from https://github.com/gpeter62/ESP_UniClock: the sketch
source, the web UI and the libraries the ESP8266 build needs, unpacked
from the bundled zips. Adds a PlatformIO project for the ESP-12. The
source is unchanged; local changes follow as separate commits.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

firmware/uniclock/lib/ArduinoJson/extras/tests/JsonSerializer/CustomWriter.cpp

@@ -0,0 +1,52 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2022, Benoit BLANCHON
+// MIT License
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+class CustomWriter {
+ public:
+  CustomWriter() {}
+
+  size_t write(uint8_t c) {
+    _str.append(1, static_cast<char>(c));
+    return 1;
+  }
+
+  size_t write(const uint8_t *s, size_t n) {
+    _str.append(reinterpret_cast<const char *>(s), n);
+    return n;
+  }
+
+  const std::string &str() const {
+    return _str;
+  }
+
+ private:
+  CustomWriter(const CustomWriter &);  // non-copiable
+  CustomWriter &operator=(const CustomWriter &);
+
+  std::string _str;
+};
+
+TEST_CASE("CustomWriter") {
+  DynamicJsonDocument doc(4096);
+  JsonArray array = doc.to<JsonArray>();
+  array.add(4);
+  array.add(2);
+
+  SECTION("serializeJson()") {
+    CustomWriter writer;
+    serializeJson(array, writer);
+
+    REQUIRE("[4,2]" == writer.str());
+  }
+
+  SECTION("serializeJsonPretty") {
+    CustomWriter writer;
+    serializeJsonPretty(array, writer);
+
+    REQUIRE("[\r\n  4,\r\n  2\r\n]" == writer.str());
+  }
+}