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

CMakeLists.txt

@@ -0,0 +1,36 @@
+cmake_minimum_required(VERSION 3.16)
+
+project(obs-streamer-tools-plugin
+    VERSION 0.0.1
+    DESCRIPTION "OBS Studio source plugin for streamer-tools camera feeds (scaffold, no LiveKit integration yet)"
+    LANGUAGES C CXX
+)
+
+set(CMAKE_C_STANDARD 11)
+set(CMAKE_C_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+enable_testing()
+
+# --- Scaffolding note ------------------------------------------------------
+# This deliberately does NOT use the full obsproject/obs-plugintemplate
+# build system (its cmake/common/bootstrap.cmake + buildspec.json, which
+# download complete OBS source archives and prebuilt dependency bundles
+# for macOS/Windows). That machinery is real and may be worth adopting
+# wholesale in a later phase; for this scaffolding pass the goal is a much
+# simpler CMakeLists.txt that proves out find_package(libobs) plus the
+# core/adapter split on the platform we can actually verify locally
+# (Linux, via the system libobs-dev package). See README.md for the full
+# writeup of what was verified vs. what remains.
+# ----------------------------------------------------------------------------
+
+add_subdirectory(core)
+
+find_package(libobs QUIET)
+if(libobs_FOUND)
+    message(STATUS "libobs found (${libobs_DIR}) -- building OBS adapter module")
+    add_subdirectory(obs-adapter)
+else()
+    message(WARNING "libobs NOT found -- skipping OBS adapter module; core library still builds/tests")
+endif()