parser: replace fprintf+longjmp with fail(), add forward declarations

Introduce a fail(p, "msg") inline function that stores the error message
in a buffer and longjmps, replacing ~52 fprintf(stderr,...)+longjmp pairs.
The error message is propagated through Ast.err_msg so callers can decide
whether/how to display it. Also add forward declarations for all static
functions and move PtrModifiers typedef to the type definitions section.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

parser.h

@@ -7,6 +7,7 @@
 #include <setjmp.h>
 #include <stdbool.h>
 #include <stdint.h>
+#include <string.h>
 
 typedef struct {
     const char* source;
@@ -22,8 +23,20 @@ typedef struct {
     AstNodeIndexSlice extra_data;
     AstNodeIndexSlice scratch;
     jmp_buf error_jmp;
+    char* err_buf;
 } Parser;
 
+#define PARSE_ERR_BUF_SIZE 200
+
+_Noreturn static inline void fail(Parser* p, const char* msg) {
+    size_t len = strlen(msg);
+    if (len >= PARSE_ERR_BUF_SIZE)
+        len = PARSE_ERR_BUF_SIZE - 1;
+    memcpy(p->err_buf, msg, len);
+    p->err_buf[len] = '\0';
+    longjmp(p->error_jmp, 1);
+}
+
 Parser* parserInit(const char* source, uint32_t len);
 void parserDeinit(Parser* parser);
 void parseRoot(Parser* parser);