test(pty): Add a terminal harness for screen tests

[JeanMertz]

A test can now assert on what a terminal renders rather than on the
bytes JP emitted. jp_pty::Terminal hands out a Writer for the code
under test to draw into and answers through Screen: what row N
holds, where the cursor is, whether a row wrapped. Scrolling, deferred
wrap at the right margin, and resize only exist on the far side of that
boundary, so none of them was reachable from a byte assertion.

Two backends sit behind one screen API. Terminal::pty opens a real pty
through portable-pty, so output crosses the kernel's line discipline,
the writer is a tty, and a child can be spawned into it, typed at, and
resized under it. Terminal::modelled feeds the vt100 screen model
directly, imitating a tty's ONLCR. Terminal::open takes the first
where the platform allows and falls back to the second, which is what
Windows gets: ConPTY is reachable only by a child process, so a
pty-only harness would have left every in-process case unix-only.
expectrl was the alternative considered and rejected: it matches a
spawned child's output stream and cannot hand a tty to in-process code.

wait_for blocks on a condvar woken by arriving bytes and returns the
screen that satisfied its predicate, so the assertions after it are made
against that snapshot rather than a later one; nothing sleeps for a
fixed interval. A timeout renders the screen with the cursor marked, and
Error's Debug defers to Display, so an unwrap in a test prints
it.