test(win32): drop the __libc_single_threaded stand-in, which crafter-build 5.0.0 made unneeded

Tests
- Win32Link linked under the Flatpak only with a C file defining
  __libc_single_threaded. crafter-build gave clang the host's /usr as
  the MinGW sysroot, which put glibc's headers on the include path, and
  libstdc++'s shared_ptr, inlined from nlohmann/json in Crafter.Asset's
  GLB loader, then read the symbol. crafter-build 5.0.0 shows clang only
  the mingw-w64 parts of /usr (Catcrafts/Crafter.Build#43), so nothing
  references it any more. Under 5.0.0 Win32Link links without it, and so
  does examples/HelloUI, which no stand-in covered. The native
  crafter-build on the host, which never had the problem, still builds
  PointerWin32, TextWin32 and Win32Link and runs them under Wine.

Docs
- AGENTS.template.md and AGENTS.consumer.md: linking for Win32 under the
  Flatpak needs crafter-build 5.0.0, and the workaround is gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

project.cpp

@@ -428,11 +428,8 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
             // no Wine, run it as --runner=cmd:true, which builds it and runs
             // nothing; no requires_, since that would skip the build too.
             // Under Wine it also runs, opening a window and closing it, which
-            // needs a Vulkan device. libc-single-threaded.c stands in for
-            // Crafter.Build #43 and goes with it.
-            Test win32LinkTest = LibraryTest("Win32Link");
-            win32LinkTest.config.cFiles.push_back("tests/Win32Link/libc-single-threaded");
-            cfg.tests.push_back(std::move(win32LinkTest));
+            // needs a Vulkan device.
+            cfg.tests.push_back(LibraryTest("Win32Link"));
         }
 
         // Regression test for issue #32: the Wayland scroll-wheel chain