Open a file from a directory listing named by a relative path

[viict]

tread ../ww-marketing-website/ lists the directory, and then every entry of
it fails to open with link escapes the index root -- with no --index
anywhere in sight. tread / fails the same way. Three defects stack up to
produce it, and each is a commit.

1. A relative path with .. did not parse

plat::path folded . and .. and treated a .. with nothing above it as
an error, because that is how a link escaping the corpus is caught. But the
same fold is used to parse a path that is merely written relatively, and
../notes/a.md names a real file. It returned None, contains said no, and
every entry of the listing was judged outside a root it was inside.

A .. with nothing above it now depends on who is asking: a relative path
keeps it, a rooted path drops it (/a/../.. is /), and joining a link onto
a base still refuses -- that caller is the one the error was written for.

2. The project search walked into the working directory

corpus_root climbed from the path's parent with Path::parent, whose answer
for .. is "" -- the working directory, which is not an ancestor of
../elsewhere at all. Run from a project, tread ../thing/ found this
project's .git and rooted the listing's corpus at a project the listing was
not in. The climb now runs on the path resolved against the cwd, folded rather
than joined, since <cwd>/../thing still holds <cwd> as a component.

3. A filesystem root fell back to .

Path::new("/").parent() is None, and the fallback was PathBuf::from("."),
so tread / rooted its corpus at wherever the reader was standing. Nothing is
above a filesystem root: the directory holding it is itself.

Verification

Reproduced first, from the reported command, through a pty:

before   CLAUDE.local.md: link escapes the index root
         tests/:          link escapes the index root
after    opens the file; `tests/` opens as a listing

before   tread /   ->  boot/: link escapes the index root
after    tread /   ->  Enter on etc/ opens `/etc  ·  177 entries  ·  [1 back]`

• 1310 unit + 90 integration tests passing, from 1307 + 90 on master.
• clippy --all-targets -D warnings clean; all five cross-targets check.
• New tests: the relative-parent listing end to end in
  src/source/dir/tests.rs (with the cwd deliberately a different project,
  which is what pins defect 2), and the .. rules per platform in
  src/plat/path_tests.rs.

walking_off_the_front_is_an_error_not_a_clamp pinned the old behaviour and is
rewritten rather than deleted: it now asserts that join still refuses an
escape, that a rooted path clamps, and that a relative one keeps its ...

Found while reading agent trajectories on another branch; this predates that
work and is independent of it.