fix: String_IsNull/String_Offset helpers + remove pointer-to-int casts

- Add String_IsNull and String_Offset wrappers in lib/String.bux
- Add bux_str_is_null and bux_str_offset runtime functions in rt/runtime.c
- Update String_Replace to use String_IsNull instead of String_Len(pos)==0
- Replace pointer-to-uint casts (as uint == 0, pointer arithmetic) across apps/
- Parser newline skipping fixes in src/parser.bux

src/parser.bux

@@ -920,6 +920,12 @@ func parserParseParamList(p: *Parser) -> *Decl {
 
     discard parserExpect(p, tkLParen, "expected '('");
     while !parserCheck(p, tkRParen) && parserPeek(p, 0) != tkEndOfFile {
+        while parserCheck(p, tkNewLine) || parserCheck(p, tkSemicolon) {
+            discard parserAdvance(p);
+        }
+        if parserCheck(p, tkRParen) || parserPeek(p, 0) == tkEndOfFile {
+            break;
+        }
         if d.paramCount >= 9 { break; }
         let nameTok: LexToken = parserExpectIdentOrKeyword(p, "expected parameter name");
         discard parserExpect(p, tkColon, "expected ':' in parameter");