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.

1 files changed, 9 insertions, 0 deletions

diff --git a/docs/shaders.md b/docs/shaders.md
index 95550f0..63801db 100644
--- a/docs/shaders.md
+++ b/docs/shaders.md
@@ -110,6 +110,15 @@ pane change, and `:paste` prints what you have as a `shaders { }` block for your
 config. What you prototype and what you paste are parsed by the same code, which
 is the point of using the config's syntax for a thing typed at a terminal.
 
+It is a readline prompt, so editing, up/down and ctrl-r work as they do in a
+shell, and history is kept between runs in
+`$XDG_DATA_HOME/sl0ppty/shader-repl.history` -- the chains only, since `:quit` is
+not something you want to press up past. Tab completes the commands, the shader
+names, the property keys and the expression language's own variables and
+functions; `:help` prints the same list at once. A test checks that list against
+`src/shader.c` and `src/expr.c`, because a completion list that has gone stale
+reads as "that is all there is".
+
 **Off by default.** It needs `in_band_shaders true`, because a program that can
 restyle the session it happens to be running in is a hazard first and a
 convenience second: `cat` the wrong file and your panes go dark. With it on, a