sl0ppty: shaders on the chrome, and two keys for the arrangement

Five things, in one commit because they land in the same few files --
the action table, the shader pass, config.kdl -- and splitting them by
hunk would buy a tidier log at the price of commits that do not build.
Each is verified end to end; the suite is 46 files green.

Chrome shaders. `where="chrome"` on any entry in `shaders` or in a
`states` block runs that pass over a pane's frame -- border, padding,
title, buttons -- instead of its contents. Nothing else changes: same
registry, same `amount`, same expressions, same chain order, one new
word, because a frame is another rect and so it is another pass. It runs
over the frame's whole rect with the contents punched out as a hole,
rather than four passes over four sides: an effect has to be able to
travel round a border, and four passes would each count x/y from zero
and restart at every corner. The hole keeps the two honest -- a skipped
cell is never visited, so a full-strength chrome tint cannot reach your
text. It is also the only kind of pass that can colour a pane a small
window has collapsed to a single row (D6), which is the state where a
colour is worth the most.

`channel="fg"|"bg"|"both"` came out of looking at the first border
flash: `tint` mixes both of a cell's colours, and a frame's background
is the terminal's own default, so the pass materialised it to our guess
at that default and mixed towards the colour -- a dark rectangle behind
the glyphs, left there until the pane changed state. The mask is
enforced by the pass, not the shader: the other colour is snapshotted
before materialising and put back after, so an unset colour goes back to
unset, it works for a built-in and a plugin alike, and no shader grows
an fg-only twin of itself.

A `bell` pane state and a `since` variable, because "flash the border
when a pane rings" needed both and neither existed. `since` is
milliseconds since the pane entered its current state, stamped in the
one function that decides the state -- a transition is the one thing a
frame cannot tell you, so it is remembered where it cannot disagree with
the state it dates. `bell` ranks above the ambient states and below
`dead`: a pane that rang while you were elsewhere is news, and a pane
that rang and then exited is a pane that exited. It ends when you look
at the pane, because that is what answering a bell is.

Animation needed a clock and did not have one: a shader reading `t` only
advanced when something else caused a frame, so a pulse froze the moment
you stopped typing. `app_next_deadline_ms` now reports `anim_ms` while a
chain that reads the clock ran over the last frame -- derived from what
was applied, so a pulse hung off `dead` costs a clock while a pane is
dead and nothing for the rest of the session. A `deadline` verb makes
that observable; the server's poll loop already lived on the number.

contrib/chrome has seven presets and the perimeter trick they are built
on: on a frame the coordinate you want is distance along the ring, which
the language can work out, and `min(d, P - d)` is what makes an effect
cross the corner it started from. Hand-written, unlike contrib/shaders,
because the toy previews contents and a frame has no preview to give --
so tests/test_chrome_presets.py checks each one the only way left: load
it into a real session, assert the frame moves and the contents do not.

`equalize` (C-a =) puts every share back to even: each split's children
weighted by how many visible panes are behind them, so one pane beside a
stack of three is a quarter of the width and "even" means the same thing
at every depth. Exactly equal cell counts are not something a tree can
always express, so the action promises the even division it can.

`rotate-layout` (C-a Space) turns the whole tab a quarter turn
clockwise. Every split changes axis and a row split's children reverse,
because the pane on top of a stack is the pane on the right once you
turn it; get that asymmetry wrong and you have a mirror image, which
looks almost right until four turns fail to come back. They do come
back, exactly, which is why the inverse is three more turns rather than
a second mirrored routine with its own chance of being wrong -- and why
a key as easy as space is the right one rather than a hazard. A turn is
refused when it would not fit: a share of the height is not always a
share the width can afford, and a node that cannot give its children
their floor takes the whole tab down to a list. So the turn is tried,
the layout is asked what it made of it, and one that flattened the tab
is put back with a word. Same rule `split_fits` applies to splitting.

Keys: the palette moves from `shift+p` to `p`, and cycling tabs from
`n`/`p` to tab/shift+tab. `p` is pressed far more often than "the tab
before this one" and was the only shifted letter in the defaults; tab is
what every other tabbed thing cycles with, and `\e[Z` decodes as
shift+tab on a plain terminal, so this costs nothing on a client without
the kitty protocol. `n` is free again. The actions are untouched: two
lines of config put the old binds back.

The shader ABI goes to 3: `shade_ctx_t` gained `state_ms` and struct
shader gained `channels`, both the pass's business rather than a
shader's. Plugins need a rebuild, which is what the version is for.

1 files changed, 8 insertions, 1 deletions

diff --git a/tests/expr_test.c b/tests/expr_test.c
index a202f06..fb191b8 100644
--- a/tests/expr_test.c
+++ b/tests/expr_test.c
@@ -23,7 +23,7 @@ static int at(const char *src, int x, int y) {
   }
   expr_env_t env = {.x = x, .y = y, .cols = 80, .rows = 24,
                     .curx = 10, .cury = 5, .cursor = 1, .focused = 1,
-                    .t = 1000};
+                    .t = 1000, .since = 250};
   int v = expr_eval(p, &env);
   expr_free(p);
   return v;
@@ -81,6 +81,12 @@ int main(void) {
   eq("cursor", 1);
   eq("focused", 1);
   eq("t", 1000);
+  eq("since", 250);
+  /* What a flash is: full strength for a moment after the state began, then
+   * nothing. `t` cannot express this — it says what time it is, not how long
+   * ago something happened. */
+  eq("(since < 300) * 255", 255);
+  eq("(since < 200) * 255", 0);
 
   printf("\n-- functions\n");
   eq("min(3, 9)", 3);
@@ -119,6 +125,7 @@ int main(void) {
         {"dist(x, y, curx, cury)", EXPR_DEP_POS | EXPR_DEP_CURSOR},
         {"focused * 90", EXPR_DEP_FOCUS},
         {"t % 1000", EXPR_DEP_TIME},
+        {"since", EXPR_DEP_TIME}, /* a clock, so never cached */
     };
     for (size_t i = 0; i < sizeof cases / sizeof *cases; i++) {
       char err[128] = {0};