_writePadded

Writes content into writer with width/alignment/fill padding applied. Used by String, integer, and float format(into:) implementations.

When fill == '0' and padding goes on the left (right-align / center), any sign character (-, +, ) or radix prefix (0b, 0o, 0x, 0X) that appears at the start of content is emitted before the zero-padding so the result matches the C printf "%08d" convention, e.g. -5 with width 8 → -0000005 rather than 000000-5.