Link the pinned LiveKit C++ SDK into the core library

Adds cmake/LiveKitSDK.cmake, adapted from
livekit-examples/cpp-example-collection's helper of the same name, with the
VERSION="latest" GitHub-API resolution path removed: this project pins an
exact client-sdk-cpp release (1.10.1, the newest tag as of today), and the
pin should not be silently bypassable. The module also now exports the
runtime shared libraries so packaging can stage liblivekit/liblivekit_ffi
next to the plugin module later.

core/ links LiveKit::livekit PUBLIC. A new smoke test proves the SDK is not
just linked but loadable and callable: livekit::initialize()/shutdown()
round-trip in-process, a second initialize() reports "already initialized",
the log level round-trips, and the SDK's generated LIVEKIT_BUILD_VERSION is
asserted equal to the version CMake pinned (so a stale extracted SDK
directory fails loudly rather than being silently reused).

Also adds core/tests/test_util.h, a dependency-free assertion harness that
keeps running after a failure and prints a pass/fail count, so CI output says
how much actually ran instead of dying on the first bare assert().

cmake_minimum_required goes 3.16 -> 3.19 (file(ARCHIVE_EXTRACT)).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RL8abRmgFXkVASHkkqiJbE

core/CMakeLists.txt

@@ -13,6 +13,11 @@ target_include_directories(stplugin_core
         ${CMAKE_CURRENT_SOURCE_DIR}/include
 )
 
+target_link_libraries(stplugin_core
+    PUBLIC
+        LiveKit::livekit
+)
+
 set_target_properties(stplugin_core PROPERTIES
     POSITION_INDEPENDENT_CODE ON
 )