Fix assertions that NDEBUG deleted, and match OBS's real macOS SDK regex

test_core.cpp used bare assert(). CI builds Release, Release defines NDEBUG,
and NDEBUG compiles assert() out entirely -- so that suite had been passing
unconditionally, checking nothing. It now uses the same always-live ST_ASSERT
harness as the other suites and reports a count (10 checks), and additionally
asserts that core_version() really is the version CMake injected rather than a
stale literal.

macOS SDK, third iteration. The previous fix assumed OBS only wanted a
version-carrying SDK filename. Reading OBS 30.0.2's
cmake/macos/compilerconfig.cmake shows the actual pattern is stricter:

  ".+/MacOSX.platform/Developer/SDKs/MacOSX([0-9]+\.[0-9])+\.sdk$"

which only ever matches a full-Xcode SDK path. A Command-Line-Tools-only
install keeps its SDK at /Library/Developer/CommandLineTools/SDKs/MacOSX<ver>.sdk,
with no MacOSX.platform/Developer/SDKs segment at all, so it can never match
however it is named -- which is why the second attempt got past the
"REGEX needs at least 5 arguments" error and still landed on "Your macOS SDK
version () is too low", with the version still empty.

_resolve_versioned_macos_sdk now builds a symlink tree under .deps/ whose
shape matches that pattern and which points at exactly the same SDK, and uses
the toolchain's own path untouched when it already matches (i.e. when real
Xcode is installed). Nothing about the compilation changes -- only the
spelling of the path, which is all OBS's check reads.

Also refreshes the scaffold-era comments in core.h and core_c.h, which still
described this library as a placeholder that would one day talk to
livekit-ffi.

Linux CI is green on the previous commit: real libobs adapter linked
(ldd shows libobs.so.0 plus liblivekit/liblivekit_ffi resolving from the
staged package directory), obs_module_load exported, ctest 6/6, artifact
uploaded.

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

core/include/stplugin/core_c.h

@@ -18,15 +18,13 @@ with this program. If not, see <https://www.gnu.org/licenses/>
 
 #pragma once
 
-// Minimal C ABI surface of the core library, for the OBS adapter (plain
-// C, per the obs-plugintemplate convention) to call into the core
-// library (C++) without needing a C++ compiler in that translation unit.
+// A minimal C ABI over the core library's version string.
 //
-// This mirrors the boundary the real integration will cross in the
-// other direction: livekit-ffi is a Rust library exposing a C ABI that
-// the C++ core library will link against. Proving a small, deliberate
-// C ABI seam works cleanly here is part of what this scaffold is for.
-
+// The OBS adapter is C++ and calls stplugin::core_version() directly, so
+// nothing in this repository needs this header today. It is kept because it
+// is the seam a plain-C consumer would use, and because the unit tests assert
+// the two entry points agree -- which is a cheap check that the C++ library
+// really is linkable from a C translation unit.
 #ifdef __cplusplus
 extern "C" {
 #endif