Replace separate float formatting (fmt_fp.c) and parsing (readfloat.c)
implementations with a unified fp_uscale.c using 128-bit unrounded
scaling. Both mrb_format_float() and mrb_read_float() now share a
single pow10 table and uscale() primitive for decimal/binary conversion.
This fixes subnormal parsing accuracy (old code returned 0.0 for the
smallest subnormals) and corrects %.2f rounding for values like
12345.125. Table size grows from ~5KB to ~11KB in .rodata.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto