sl0ppty: a prompt with a memory

`contrib/shader-repl` imports readline, which is most of it: editing,
up/down, ctrl-r, and history kept between runs in
`$XDG_DATA_HOME/sl0ppty/shader-repl.history`. Chains only -- a `:` command
is taken back out of the history it would otherwise fill, because pressing
up four times past `:quit` and `:paste` to reach the line you were working
on is the annoyance history exists to remove.

Tab completes the commands, the nine shader names, the property keys, the
two rects, the three channels, and the expression language's own variables
and functions. `:help` prints the lot at once, which is the same list from
the same place.

That list is a claim about what exists, so `tests/test_shader_repl.py`
checks it against `src/shader.c` and `src/expr.c` and fails if either grows
a name it does not have -- verified by removing `spotlight` and watching it
fail. Then it types every name it offers at a real prompt and requires `ok`
from a real session, because a name that completes and does not parse is a
lie with a keystroke attached. History across two runs, a tab completion,
and `:help` are driven the same way, through a pty.

Two things that cost me time and are now written down where they bite:
keys go into a pane escaped (`\r`, `\t`, `\e[A`) because `raw` takes the rest
of one command line and a literal newline ends the command instead of
arriving; and Enter is CR, which is what a terminal sends and what readline
waits for.

Also: **a bare argument on a shader entry is now a complaint.** Every
parameter is a property, so `tint color="#000000" spotlight` is not a pass
with a spare word, it is a missing `;` -- KDL reads one node, the second
name does nothing, and the session used to answer `ok`. It now says
`unexpected argument for tint: spotlight (separate entries with ';')`. Found
by watching the prompt get used.

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