ci: build the real OBS adapter on all three platforms

Two things: unbreak the Windows compile, and give macOS/Windows a libobs.

MSVC fix. test_json.cpp failed to compile on the Windows runner with
"a universal-character-name specifies an invalid character" and "illegal
escape sequence" -- MSVC still forms escape sequences and
universal-character-names INSIDE raw string literals, which it must not.
Every JSON input containing a backslash is now built by string concatenation
from a single kBS constant, which also sidesteps the separate murky corner of
translation phase 1 where a doubled backslash immediately followed by 'u' has
historically been treated inconsistently. Same 158 checks, no behaviour
change.

OBS SDK bootstrap for macOS/Windows. Adopts obsproject/obs-plugintemplate's
buildspec machinery -- buildspec.json plus cmake/common/buildspec_common.cmake
and cmake/{macos,windows}/buildspec.cmake -- so those two platforms get a real
libobs and build the actual plugin module instead of only the core library.
Linux is untouched and still uses Ubuntu's libobs-dev
(-DSTPLUGIN_BOOTSTRAP_OBS=OFF); the bootstrap only runs where there is no
system package.

Trimmed against upstream, each change recorded in the file that makes it:

 - qt6 is dropped from dependencies_list on both platforms. The properties UI
   is plain obs_properties_* and nothing here links Qt.
 - The OBS sub-build builds and installs the `libobs` target, not
   `obs-frontend-api`. Building the frontend API is what would drag Qt back in,
   and this plugin never calls it.
 - The sub-build is configured with ENABLE_UI=OFF and ENABLE_SCRIPTING=OFF as
   well as upstream's ENABLE_FRONTEND=OFF: the pinned OBS predates
   ENABLE_FRONTEND and gates its Qt-dependent UI on ENABLE_UI, so without this
   it configures the whole OBS UI and demands Qt anyway.
 - Only the Release configuration is built and installed, not Debug as well.
   Nothing consumes a debug libobs and it doubles the slowest CI step.
 - Only the dependency-acquisition modules are vendored. The template's
   compilerconfig/defaults/helpers/xcode modules drive its own target and
   bundle layout, which this project does not use.

obs-studio is pinned to 30.0.2, deliberately low: OBS refuses to load a module
built against a NEWER libobs than the one running it and accepts older ones, so
this pin IS the minimum OBS version users need. 30.0.2 is also exactly what
Ubuntu 24.04's libobs-dev ships, which puts all three platforms on one floor,
and it supports the modern CMake layout the bootstrap drives via
-DOBS_CMAKE_VERSION=3.0.0. prebuilt is obs-deps 2023-11-03 with the hashes
obs-studio 30.0.2's own buildspec.json publishes; the obs-studio source
archive hashes were computed from the GitHub tag archives.

The workflow also prints what was actually produced on each platform (ldd /
otool / dir over build/package) and uploads it as an artifact, so "does this
even link against libobs" is answered by CI output rather than assumed.

Verified locally: the Linux path is unchanged by all of this -- a fresh
configure still finds libobs-dev, and ctest is 6/6. The macOS and Windows
bootstrap can only be verified by CI; that is what this push is for.

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

CMakeLists.txt

@@ -18,6 +18,37 @@ endif()
 enable_testing()
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/common")
+
+# --- OBS SDK ---------------------------------------------------------------
+# Linux gets libobs from the distribution (Ubuntu's libobs-dev ships real
+# libobsConfig.cmake), and that path is left exactly as it was.
+#
+# macOS and Windows have no such package, so they use obs-plugintemplate's
+# buildspec bootstrap, trimmed: it downloads the pinned obs-deps bundle and
+# the pinned obs-studio source, then builds and installs just `libobs`. See
+# buildspec.json for why the OBS pin is deliberately low, and
+# cmake/common/buildspec_common.cmake for every change from upstream.
+#
+# STPLUGIN_BOOTSTRAP_OBS=OFF falls back to plain find_package(libobs), for a
+# developer who already has an OBS SDK on their prefix path and does not want
+# a from-source libobs build.
+option(STPLUGIN_BOOTSTRAP_OBS "Download and build libobs from source (macOS/Windows)" ON)
+
+include(osconfig)
+if(STPLUGIN_BOOTSTRAP_OBS AND (OS_MACOS OR OS_WINDOWS))
+    if(OS_MACOS)
+        # client-sdk-cpp ships single-arch dylibs, so this plugin is built for
+        # one architecture even though the libobs it links is universal.
+        if(NOT CMAKE_OSX_ARCHITECTURES)
+            set(CMAKE_OSX_ARCHITECTURES "${CMAKE_HOST_SYSTEM_PROCESSOR}" CACHE STRING "" FORCE)
+        endif()
+        if(NOT CMAKE_OSX_DEPLOYMENT_TARGET)
+            set(CMAKE_OSX_DEPLOYMENT_TARGET "13.0" CACHE STRING "" FORCE)
+        endif()
+    endif()
+    include(buildspec)
+endif()
 
 # --- LiveKit C++ client SDK -------------------------------------------------
 # Pinned, prebuilt release of livekit/client-sdk-cpp, downloaded and unpacked