printf "%c" format specifier doesn't work in batch mode

```
color C = 0.5;
printf("C = %c\n", C);
```

works in single point mode, not in batch mode. Changing to `%g` works for both.

Reproduce the provided printf example in single-point and batch modes, comparing the %c and %g format specifiers. Trace the batch-mode printf handling and make the behavior consistent with single-point mode; done means %c prints the value correctly in both modes.

Written by the indexing model from the issue text.