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, 11 insertions, 6 deletions

diff --git a/src/expr.h b/src/expr.h
index 075df7d..6508ec7 100644
--- a/src/expr.h
+++ b/src/expr.h
@@ -20,15 +20,18 @@
  * nothing in here has an effect. Evaluation is therefore O(program length),
  * a bad expression cannot hang a session, and there is nothing to sandbox.
  *
- *   variables   x y cols rows curx cury cursor focused t
+ *   variables   x y cols rows curx cury cursor focused t since
  *   operators   + - * / %   < > <= >= == !=   && || !   ?:
  *   functions   min max abs clamp dist
  *
- * `x`/`y` are cell positions inside the pane's content rect, `t` is a
- * millisecond clock for animation, `dist` corrects for a cell being about
- * twice as tall as it is wide. Division and modulo by zero are 0 rather than
- * a trap: an expression that is wrong should look wrong, not take the session
- * with it.
+ * `x`/`y` are cell positions inside the rect the pass runs over (a pane's
+ * contents, or its frame for a chrome pass), `t` is a millisecond clock for
+ * animation, `since` is how many milliseconds the pane has been in the state
+ * it is in — which is what a one-shot effect needs, because `t` says what time
+ * it is and not how long ago something happened. `dist` corrects for a cell
+ * being about twice as tall as it is wide. Division and modulo by zero are 0
+ * rather than a trap: an expression that is wrong should look wrong, not take
+ * the session with it.
  */
 #ifndef SL0PPTY_EXPR_H
 #define SL0PPTY_EXPR_H
@@ -71,6 +74,8 @@ typedef struct {
   int curx, cury, cursor;
   int focused;
   int64_t t;
+  /* Milliseconds since the pane entered its current state, clamped at 0. */
+  int64_t since;
 } expr_env_t;
 
 /* Evaluate for one cell. Clamped to 0..255 by the caller, not here: the