refactor: batch 1 - correctness and error contract

- MID/RIGHT/LEFT integer args now coerced at call sites (RIGHT(s, n/2)
  no longer emits invalid IR); drop emitLeft's private conversion
- FOR loop direction decided at run time via select on STEP sign;
  descending FOR works, STEP evaluated once per iteration
- ArrayHandler: extract emitCheckedIndices/loadElement; index count is
  validated before emission, fixing a compiler-crash (vector OOB UB) on
  OUTPUT with too many indices
- Error contract: Lexer/Parser/CodeGen track errors; all silent failure
  paths now print Error: diagnostics with line numbers; unterminated
  char literal no longer truncates the rest of the program; module is
  verified with verifyModule; driver exits non-zero on any error

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/include/cps/Parser.h b/include/cps/Parser.h
index 19f49be..87d1dee 100644
--- a/include/cps/Parser.h
+++ b/include/cps/Parser.h
@@ -4,6 +4,7 @@
 #include <vector>
 #include <memory>
 #include <map>
+#include <optional>
 #include <tuple>
 #include <string>
 
@@ -12,7 +13,8 @@ namespace cps {
 class Parser {
     Lexer &Lex;
     int CurTok;
-    
+    unsigned NumErrors = 0;
+
     std::map<int, int> BinopPrecedence;
 
     int getNextToken();
@@ -31,6 +33,7 @@ class Parser {
     std::unique_ptr<ExprAST> ParseStringBuiltin(const std::string &FuncName);
 
     std::unique_ptr<StmtAST> ParseStatement();
+    std::unique_ptr<StmtAST> ParseStatementImpl();
     std::unique_ptr<StmtAST> ParseIfStmt();
     std::unique_ptr<StmtAST> ParseWhileStmt();
     std::unique_ptr<StmtAST> ParseRepeatStmt();
@@ -42,11 +45,12 @@ class Parser {
     std::unique_ptr<StmtAST> ParseProcedure();
     std::unique_ptr<StmtAST> ParseCallStmt();
     std::unique_ptr<StmtAST> ParseReturnStmt();
-    std::vector<std::tuple<std::string, std::string, bool>> ParsePrototypeArgs();
-    
+    std::optional<std::vector<std::tuple<std::string, std::string, bool>>> ParsePrototypeArgs();
+
 public:
     Parser(Lexer &L);
     std::vector<std::unique_ptr<StmtAST>> Parse();
+    bool hadError() const { return NumErrors != 0; }
 };
 
 } // namespace cps