feat(runtime): write runtime diagnostics to stderr

Runtime check messages and the unhandled-exception message were printed
to stdout by the generated program, so `cpsc x.cps -r >out.txt` captured
them. They are now formatted with snprintf and written with write(2).
Test expectations and docs are updated accordingly.

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

1 files changed, 8 insertions, 2 deletions

diff --git a/include/cps/RuntimeCheck.h b/include/cps/RuntimeCheck.h
index f5a8aa7..8f33e99 100644
--- a/include/cps/RuntimeCheck.h
+++ b/include/cps/RuntimeCheck.h
@@ -14,7 +14,10 @@ class RuntimeCheck {
     llvm::LLVMContext &TheContext;
     llvm::IRBuilder<> &Builder;
 
-    llvm::FunctionCallee PrintfFunc;
+    llvm::FunctionCallee SnprintfFunc;
+    llvm::FunctionCallee WriteFunc;
+    llvm::FunctionCallee StrLenFunc;
+    llvm::GlobalVariable *ErrBuf;
     ExceptionHandler &Exceptions;
     llvm::Value *DivZeroMsg;
     llvm::Value *OutOfBoundsMsg;
@@ -29,6 +32,9 @@ public:
     RuntimeCheck(llvm::Module &M, llvm::LLVMContext &C, llvm::IRBuilder<> &B,
                  ExceptionHandler &EH);
 
+    // Format a runtime diagnostic (printf-style) and write it to stderr.
+    void emitStderr(llvm::Value *Msg, llvm::ArrayRef<llvm::Value *> FmtArgs = {});
+
     void emitDivZeroCheck(llvm::Value *Divisor, int Line);
     void emitIndexCheck(llvm::Value *Index, llvm::Value *Lower, llvm::Value *Upper, int Line);
 
@@ -44,7 +50,7 @@ public:
     void emitArrayArgBoundsCheck(llvm::Value *ActualLower, llvm::Value *ActualUpper,
                                  int64_t DeclaredLower, int64_t DeclaredUpper, int Line);
 
-    // Print the diagnostic, then throw an anonymous exception on failure.
+    // Print the diagnostic to stderr, then throw an anonymous exception on failure.
     // Runtime-computed variant: FmtArgs are the printf varargs after Msg
     // (e.g. {i32 line, ptr filename}). Needed when the failure site is inside
     // a synthesized helper function, where the line number is an argument