parser: propagate errors via setjmp/longjmp instead of exit(1)

Replace 32 parse-error exit(1) calls with longjmp to allow callers to
detect and handle parse failures. The OOM exit(1) in
astNodeListEnsureCapacity is kept as-is.

Add has_error flag to Ast, wrap parseRoot() with setjmp in astParse(),
and update test infrastructure to use the C parser for testError tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

parser.h

@@ -4,6 +4,7 @@
 
 #include "ast.h"
 #include "common.h"
+#include <setjmp.h>
 #include <stdbool.h>
 #include <stdint.h>
 
@@ -20,6 +21,7 @@ typedef struct {
     AstNodeList nodes;
     AstNodeIndexSlice extra_data;
     AstNodeIndexSlice scratch;
+    jmp_buf error_jmp;
 } Parser;
 
 Parser* parserInit(const char* source, uint32_t len);