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

rt/runtime.c

@@ -232,11 +232,22 @@ const char* bux_strstr(const char* haystack, const char* needle) {
     return strstr(haystack, needle);
 }
 
+/* String offset: byte offset of pos within base (both must point into same string) */
+unsigned int bux_str_offset(const char* pos, const char* base) {
+    if (!pos || !base) return 0;
+    return (unsigned int)(pos - base);
+}
+
 /* String contains: returns 1 if haystack contains needle, 0 otherwise */
 int bux_str_contains(const char* haystack, const char* needle) {
     return bux_strstr(haystack, needle) != NULL;
 }
 
+/* String is null: returns 1 if string is NULL */
+int bux_str_is_null(const char* s) {
+    return s == NULL;
+}
+
 /* String slice: extract substring from start, length len */
 char* bux_str_slice(const char* s, unsigned int start, unsigned int len) {
     if (!s) return NULL;