dir: implement it

[metif12]

dir briefly lists directory contents, with -a, -C, -1, -R, -r, -S, -t, -w and the long -- spellings.

Listing and sorting live in common/dir; src/dir is a thin main.

-R prefixes each subdirectory with its path and separates it with a blank line, descending after its parent has been printed rather than depth-first.

Three behaviours were wrong on the first attempt and were caught by comparison rather than by reading: the default layout is columns separated by two spaces, not one line per entry; -a has to synthesise the . and .. entries because os.ls omits them; and a path that cannot be accessed exits 2 rather than 1. All three are pinned by tests.

A control run confirms the suite has teeth: breaking the column separator from two spaces to one turns test_default_is_columns_not_one_per_line, test_all_includes_dot_entries and test_reverse red.

One deliberate divergence, and the reason it is worth a reviewer's attention. GNU and uutils disagree about -S. On a tree of two 2-byte files and a directory, GNU orders sub one.txt two.txt — directories first, because a directory reports a larger size — while uutils on Windows orders one.txt two.txt sub. Both were measured on the same tree, GNU through WSL. This port follows GNU and test_size_sort_puts_directories_first pins that order with the disagreement written into the test.

Multi-column wrapping when the names do not fit the line width is not implemented; the single-line case is what the reference and the tests cover.