test(sysroot): cover the std module source choice for a sysroot build

SysrootStdModule runs over staged layouts that need no compiler: a
sysroot with its own module source, one on a point release of the
toolchain's libc++, one on the next major release with a shipped
source, one nothing was shipped for, one without libc++ at all, and the
two cases where a release cannot be read. The last block drives Build()
against a sysroot claiming libc++ 99 and expects the one-line
explanation back instead of compiler output, which is the failure the
fix replaces.

ToolchainResolve additionally checks that the release is read from the
libc++ the module source comes from (the prefix's, an explicit libc++
install's, the system's) and where the shipped sources are looked up.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

project.cpp

@@ -129,6 +129,7 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
         cfg.AddTest("TestRunnerSpec").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("VariantId").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("ToolchainResolve").Dependencies({ CrafterBuildLib.get() });
+        cfg.AddTest("SysrootStdModule").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("VersionFlag").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("WasiBrowserRuntime").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("WasmVariants").Dependencies({ CrafterBuildLib.get() });