--summary human sizes: align number/suffix columns + --summary-precision

[helly25]

The --summary human size column right-aligned each N.N XiB string as a whole, so units were ragged and decimal points didn't line up. This splits the size into two aligned columns.

Before

log    1  2.4 MiB
md     1      2 B
txt    2  1.5 KiB
total  4  2.4 MiB

After (numbers right-aligned with a fixed fraction area so points line up; suffixes left-aligned, all starting at one column; exact bytes render as the integer with the fraction columns blanked)

log    1  2.38 MiB
md     1  2    B
txt    2  1.47 KiB
total  4  2.39 MiB

• Bytes render as 12 B (integer, fraction area blanked to keep the point aligned), not the 12.00 B "cringe".
• New --summary-precision=N sets the scaled-unit fraction digits (default 2, matching the 123.45 target; 0 drops the point). --human=off (raw grouped bytes) is unchanged.

format::SizeColumns(bytes, units, fraction_digits) returns the {number, suffix} split (sharing a new ScaleSize with format::Size); the summary renderer pads the number column to its widest cell and appends " " + suffix.

Self-doc: the --summary-precision GlobalFlag (so --help/--man/--markdown pick it up). Tests: format_test (split, byte blanking, zero precision), summary_test (byte spacing + precision control), and the summary_additive golden regenerated. Full bazel test //xff/... green (49); both clang-format versions + shfmt/shellcheck + pre-commit clean.

Note: this makes the aligned layout the default for --summary (the old ragged format is replaced, not kept as a toggle) since --summary is always tabular. Easy to gate behind a flag instead if you'd prefer.