du: implement it

[metif12]

du for recursive apparent-size summaries: one line per directory with its total, children printed before their parent, plus the usual modifiers.

The walk and the formatting live in common/du, with src/du an eleven-line main, for the same reason df's logic does: v test <dir> compiles only _test.v files, so a test beside the implementation cannot see it.

Options: -a -b -B -c -d -h -H -k -m -s --max-depth --total --apparent-size and file operands. Clusters like -sb and -ab are parsed directly, since common.flag_parser does not expand them.

• Traversal is sorted, so output is deterministic instead of depending on directory order.
• A missing path prints du: cannot access <p>: No such file or directory, continues with the remaining operands, and exits 1, which is what GNU does.
• Block sizes round up: a path of one byte is one block, not zero.

Measured against GNU 9.4 before writing the code, including -d0, -d1, -a, -s, -c, multiple operands, and the --apparent-size -k form.

Two notes on things I did not share:

human_readable is duplicated from common/df rather than imported. An earlier revision imported it, which stacked this PR on the df one; a base branch in a fork cannot back a pull request here, so that draft could not be built at all. Keeping the two tools independent matters more than the ~25 lines, and the duplication is called out in a comment so it can be folded once both land.

Two documented divergences. The default reports apparent size in 1K blocks, which is GNU's --apparent-size accounting rather than its st_blocks allocation — V's os.Stat exposes no blocks field, so matching GNU here needs a hand-declared stat struct per platform for a difference that only shows on small files. And --block-size accepts plain decimal only.

uutils 0.0.17's du on Windows reports 0 for every block count, omits "cannot access" from its diagnostics, and exits 0 when a path is missing. None of those are copied; the tests pin GNU's behaviour instead.

20 test functions, all green, including a control that confirmed they fail when the block rounding is broken.