Scaffold core/OBS-adapter split, prove CMake toolchain, add 3-platform CI

First pass on the streamer-tools OBS camera plugin: a minimal but real
CMake project matching the design doc's core-library/OBS-adapter split
(docs/superpowers/specs/2026-09-06-obs-camera-plugin-design.md in the
streamer-tools repo). No LiveKit FFI integration yet -- this proves the
toolchain works.

- core/: dependency-free C++17 library (no OBS dependency), unit tested
  via CTest with no external test framework.
- obs-adapter/: adapted from obsproject/obs-plugintemplate (commit
  3e7d7ac, 2025-12-09). Registers a real, stubbed OBS source type;
  builds as a genuine dynamically-linked OBS module against Ubuntu's
  system libobs-dev (confirmed via ldd/nm, not a fake stand-in).
- Simpler hand-written top-level CMakeLists.txt in place of the
  template's full buildspec-driven bootstrap (which downloads full OBS
  source + prebuilt deps) -- find_package(libobs) alone is enough on
  Linux; falls back to core-library-only when libobs isn't found
  (expected on macOS/Windows CI for now).
- .gitea/workflows/build.yml: 3-platform matrix (ubuntu-latest,
  macos-latest, windows-latest) matching the runners confirmed
  available to this repo under the CyberCoveLLC org.

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

core/tests/CMakeLists.txt

@@ -0,0 +1,7 @@
+add_executable(stplugin_core_tests
+    test_core.cpp
+)
+
+target_link_libraries(stplugin_core_tests PRIVATE stplugin_core)
+
+add_test(NAME stplugin_core_tests COMMAND stplugin_core_tests)

core/tests/test_core.cpp

@@ -0,0 +1,59 @@
+/*
+streamer-tools OBS Camera Plugin - core library tests
+Copyright (C) 2026 CyberCoveLLC <jknapp85@gmail.com>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along
+with this program. If not, see <https://www.gnu.org/licenses/>
+*/
+
+// Deliberately dependency-free (no gtest/catch2 etc.) so this test target
+// has no network fetch or package-manager step in CI -- proving the
+// "core library builds and tests headlessly, no OBS required" claim
+// without adding another moving part to this scaffolding pass.
+
+#include <cassert>
+#include <cstdio>
+#include <cstring>
+
+#include "stplugin/core.h"
+#include "stplugin/core_c.h"
+
+int main() {
+    // core_version() should return a non-empty string, via both the
+    // C++ API and the C ABI wrapper the OBS adapter will actually call.
+    const char *cpp_version = stplugin::core_version();
+    assert(cpp_version != nullptr);
+    assert(std::strlen(cpp_version) > 0);
+
+    const char *c_version = stplugin_core_version();
+    assert(c_version != nullptr);
+    assert(std::strcmp(cpp_version, c_version) == 0);
+
+    // ConnectionConfig::is_valid() -- trivial non-empty checks, but
+    // exercised through all four combinations to prove the core library
+    // is genuinely testable in isolation.
+    stplugin::ConnectionConfig valid{"https://streamers.example.com", "main-room", "readkey123"};
+    assert(valid.is_valid());
+
+    stplugin::ConnectionConfig missing_url{"", "main-room", "readkey123"};
+    assert(!missing_url.is_valid());
+
+    stplugin::ConnectionConfig missing_slug{"https://streamers.example.com", "", "readkey123"};
+    assert(!missing_slug.is_valid());
+
+    stplugin::ConnectionConfig missing_key{"https://streamers.example.com", "main-room", ""};
+    assert(!missing_key.is_valid());
+
+    std::printf("core: all tests passed\n");
+    return 0;
+}