feat: add application API documentation and C++ SDK

- Add complete API documentation for application integration (docs/API_DOCUMENT.md)
- Fix check-update API to use version ID comparison instead of string comparison
- Add validation: client version must exist in server version list
- Add support for patch/incremental updates with base_version check
- Add C++ SDK with HTTP client, JSON parser, and crypto support
- Add simple_test.cpp for standalone testing on Windows

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

sdk/cpp/vcpkg.json

@@ -0,0 +1,10 @@
+{
+  "name": "verify-sdk",
+  "version": "1.0.0",
+  "description": "C++ SDK for Verify Platform",
+  "dependencies": [
+    "curl",
+    "nlohmann-json",
+    "openssl"
+  ]
+}