address gcc 16.1.0 warning: variable set but not used

[kzc]

../src/lang/fmt.c: In function 'fmt_write_frag_set_dbg':
../src/lang/fmt.c:345:18: warning: variable 'i' set but not used [-Wunused-but-set-variable=]
  345 |         uint32_t i;
      |                  ^

[kzc]

This warning was addressed by a recent commit.