refactor(runtime): write runtime diagnostics with fprintf(stderr)

Replace snprintf+write(2) with a single fprintf call on the platform's
stderr: the C library's stderr / __stderrp, or __acrt_iob_func(2) on
Windows, which the embedded runner binds to the host shim.

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

1 files changed, 3 insertions, 4 deletions

diff --git a/include/cps/RuntimeCheck.h b/include/cps/RuntimeCheck.h
index 8f33e99..30a1910 100644
--- a/include/cps/RuntimeCheck.h
+++ b/include/cps/RuntimeCheck.h
@@ -14,10 +14,9 @@ class RuntimeCheck {
     llvm::LLVMContext &TheContext;
     llvm::IRBuilder<> &Builder;
 
-    llvm::FunctionCallee SnprintfFunc;
-    llvm::FunctionCallee WriteFunc;
-    llvm::FunctionCallee StrLenFunc;
-    llvm::GlobalVariable *ErrBuf;
+    llvm::FunctionCallee FprintfFunc;
+    llvm::FunctionCallee StderrFunc;   // Windows: __acrt_iob_func(2)
+    llvm::GlobalVariable *StderrVar;   // elsewhere: the C library's stderr
     ExceptionHandler &Exceptions;
     llvm::Value *DivZeroMsg;
     llvm::Value *OutOfBoundsMsg;