fix(selfhost): resolve segfault in buxc2 compiling src/

- Replace Decl's 256 inline struct fields with dynamic fields: *StructField
  array to avoid parser field limit truncation and name shifts.
- Update parser.bux and hir_lower.bux to use decl.fields[i].
- Fix uninitialized Symbol.refType in three stack-allocated Symbol sites:
  loopSym (for-loop iterator), vSym (enum variants), and HIR param sym.
  Garbage refType values (e.g. 0xFF) were leaking into scope tables,
  later copied to expr->refType, causing SIGSEGV in Lcx_LowerExpr.

src/lexer.bux

@@ -323,6 +323,11 @@ func lexScanIdent(lex: *Lexer) {
         lex.tokens[lex.tokenCount] = tok;
         lex.tokenCount = lex.tokenCount + 1;
     }
+    if String_StartsWith(tok.text, "field") {
+        PrintLine(String_Concat("DEBUG lexer ident=", tok.text));
+        PrintLine(String_Concat("DEBUG lexer addr=", bux_int_to_str(tok.text as int64)));
+        PrintLine(String_Concat("DEBUG lexer pos=", bux_int_to_str(lex.tokenCount as int64)));
+    }
 }
 
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