fix(ci): add upstream clang to test-run env for macOS

On macOS runners without upstream clang in PATH, clice falls back to
/usr/bin/clang++ (Apple's) which emits vendor-specific flags that
upstream LLVM 21 cannot parse, causing 28 unit test failures on
macos-15-intel. Add clang/clangxx to the test feature for macOS
platforms so test-run gets upstream clang.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 16bit-ykiko

pixi.lock

@@ -81,6 +81,18 @@ scripts = ["scripts/activate_cross_linux.sh"]
 [feature.test.dependencies]
 python = ">=3.13"
 
+# clice discovers toolchain flags by running `clang++ -###`. On macOS, the
+# system Apple clang emits vendor-specific flags that upstream LLVM cannot
+# parse, causing unit test failures.  Providing upstream clang in PATH
+# prevents fallback to /usr/bin/clang++.
+[feature.test.target.osx-64.dependencies]
+clang = "==20.1.8"
+clangxx = "==20.1.8"
+
+[feature.test.target.osx-arm64.dependencies]
+clang = "==20.1.8"
+clangxx = "==20.1.8"
+
 [feature.test.pypi-dependencies]
 pytest = "*"
 pytest-asyncio = ">=1.1.0"