docs(toolchain): clarify MINGW64 LLVM and Clang 23 setup

Select the MINGW64 compiler executables with explicit Windows paths and
add version checks for installed Clang and downloaded LLVM packages.
Document Windows PATH requirements and platform-specific C++ driver names.

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

1 files changed, 10 insertions, 2 deletions

diff --git a/AGENTS/ARCH.md b/AGENTS/ARCH.md
index 178a8a6..ccbd9bf 100644
--- a/AGENTS/ARCH.md
+++ b/AGENTS/ARCH.md
@@ -34,7 +34,8 @@ uses C++ promotions, except that / always returns pseudocode REAL.
 
 The compiler emits textual IR. Initial configuration must explicitly select
 `CPSC_RUN_MODE=PATH`, `EMBEDDED` or `NONE` (no default). All modes need LLVM
-development libraries to build; NONE still generates IR. Native binaries remain manual:
+development libraries to build; NONE still generates IR. Native binaries remain manual.
+The following commands use Debian/Ubuntu's versioned compiler names:
 
 ```bash
 # Prepare dependencies using the recommendations in doc/build.md.
@@ -69,7 +70,8 @@ not to CMake. The Windows Pandoc ZIP is downloaded directly from its official
 release because MSYS2 has no Pandoc package. Only the Linux build is verified.
 
 Generated IR uses the C++ exception ABI; native binaries must link its runtime
-(use `clang++-23`). TRY/CATCH/ENDTRY and bare THROW are compiler extensions.
+(use LLVM 23's `clang++`, or `clang++-23` on Debian/Ubuntu).
+TRY/CATCH/ENDTRY and bare THROW are compiler extensions.
 Runtime checks throw anonymous exceptions; the final handler in generated main
 prints `Unhandled exception. Program terminated.` to stdout and returns 1.
 Exception execution is verified on Linux with LLVM 23; Windows/macOS are unverified.
@@ -117,6 +119,12 @@ builds link the C++ runtime statically; MinGW uses `-static`. The tested Debian
 LLVM 23 executable has only libc, libm and the system loader as dynamic dependencies.
 Windows EMBEDDED is untested.
 
+Windows build instructions retain MSYS2 MINGW64. Its LLVM/Clang package names
+and compiler executable names are unversioned; CMake still requires major 23.
+The build guide explicitly selects `/mingw64/bin/clang.exe` and `clang++.exe`
+through `cygpath -m`. MINGW64 Clang retains the GNU linker and C++ library;
+switching to CLANG64 would also change the C/C++ runtime and is a separate task.
+
 On Linux, `make -C build install` builds the manual and calls the POSIX
 `install.sh` to copy `cpsc` into `CMAKE_INSTALL_PREFIX/bin` and all manuals into
 `CMAKE_INSTALL_PREFIX/share/cpsc/man/man1` and `man7` (default prefix `/usr/local`).