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/include/stplugin/core.h

@@ -0,0 +1,57 @@
+/*
+streamer-tools OBS Camera Plugin - core library
+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/>
+*/
+
+#pragma once
+
+#include <string>
+
+// C++ API for the core library. Per the design doc
+// (docs/superpowers/specs/2026-09-06-obs-camera-plugin-design.md in the
+// streamer-tools repo), this library will eventually own: streamer-tools
+// API auth, LiveKit FFI session management (connect, subscribe, decode,
+// reconnect), and frame callbacks -- all with zero OBS dependency, so it
+// can be built and tested headlessly.
+//
+// THIS IS SCAFFOLDING. Nothing below talks to a real server or to
+// livekit-ffi yet. It exists to prove the core-library/OBS-adapter split
+// builds, links, and is unit-testable, ahead of a later phase that
+// implements the real logic.
+
+namespace stplugin {
+
+// Returns the core library's version string. Placeholder for a real
+// version scheme once the library does something.
+const char *core_version();
+
+// Minimal connection configuration the future core library will use to
+// authenticate against the streamer-tools API
+// (see apps/server/src/rooms/join.routes.ts and
+// apps/server/src/livekit/tokens.ts in the streamer-tools repo for the
+// existing read-key-authed token pattern this will follow) and mint a
+// scoped LiveKit subscriber token. Validation here is intentionally
+// trivial -- it exists to prove the core library is unit-testable
+// headlessly, not to implement the real API client.
+struct ConnectionConfig {
+    std::string server_url;
+    std::string room_slug;
+    std::string read_key;
+
+    bool is_valid() const;
+};
+
+} // namespace stplugin