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

obs-adapter/CMakeLists.txt

@@ -0,0 +1,37 @@
+# streamer-tools OBS Camera Plugin - OBS adapter
+#
+# Thin glue only, per the design doc: source registration, (eventually)
+# properties UI, and pushing frames into OBS. All real logic lives in
+# ../core. Only added to the build when find_package(libobs) succeeds
+# (see top-level CMakeLists.txt) -- see README.md for why.
+
+set(STPLUGIN_PROJECT_NAME "streamer-tools-camera")
+set(STPLUGIN_PROJECT_VERSION "${PROJECT_VERSION}")
+
+configure_file(
+    src/plugin-support.c.in
+    ${CMAKE_CURRENT_BINARY_DIR}/plugin-support.c
+    @ONLY
+)
+
+add_library(${STPLUGIN_PROJECT_NAME} MODULE
+    src/plugin-main.c
+    ${CMAKE_CURRENT_BINARY_DIR}/plugin-support.c
+)
+
+target_include_directories(${STPLUGIN_PROJECT_NAME}
+    PRIVATE
+        ${CMAKE_CURRENT_SOURCE_DIR}/src
+        ${CMAKE_CURRENT_BINARY_DIR}
+)
+
+target_link_libraries(${STPLUGIN_PROJECT_NAME}
+    PRIVATE
+        OBS::libobs
+        stplugin_core
+)
+
+set_target_properties(${STPLUGIN_PROJECT_NAME} PROPERTIES
+    PREFIX ""
+    OUTPUT_NAME ${STPLUGIN_PROJECT_NAME}
+)