slosh: defaults most people can live in, said by shaders

The out-of-the-box look, rethought as one piece:

- the compiled-in palette calms down: muted blue #7aa2f7 on the same
  neutral darks, with the colours that mean something kept semantic --
  amber bell, red-orange dead. sl0p stays one include away as the house
  style, and contrib/themes/default.kdl is the starting point written
  down (theme-tour shows it).

- derived state defaults now actually follow the theme: the scrolled
  wash is rebuilt from scroll_bg after the theme block (it used to bake
  the compiled accent), and declaring a state -- in any file of an
  include chain -- still wins. The dump writes derived states as
  comments so C-a e cannot pin what is meant to keep following.

- a bell you can see: the frame blinks three times in theme's bell
  colour then breathes until answered, and a soft diagonal shimmer --
  the bell colour halved towards default_fg, width breathing on a sine
  -- crosses the body for the same 900ms. bell_indicator false silences
  all of it; states { bell { } } replaces it like any state. since
  carries the clock dependency, so an idle session pays nothing.

- scrollback says where it continues: while scrolled, the viewport's
  top and bottom rows melt towards an edge with content past it and
  turn solid at the buffer's end. New expression variables above/below
  (lines hidden past each edge, own dependency bit, cached per scroll
  position), carried in shade_ctx_t -- SLOSH_SHADER_ABI 3 -> 4.

- expressions survive the dump: a program keeps its source and cb_chain
  writes it back, so amount="(y % 2) * 40" no longer dumps as a number.

- headers move to include/, -Iinclude everywhere; the plugin Makefile's
  SLOSH var becomes SLOSH_DIR because slosh exports SLOSH=1 into every
  pane it runs, and building from inside a session read that as a path.

- comment drift culled where features outran the prose, and the
  derivation boilerplate folded into derived_pass()/ctx_scroll().

3 files changed, 19 insertions, 4 deletions

diff --git a/docs/chrome.md b/docs/chrome.md
index a348fcc..5262b18 100644
--- a/docs/chrome.md
+++ b/docs/chrome.md
@@ -15,6 +15,13 @@ Nothing else changes: same shaders, same `amount`, same
 [expressions](shaders.md#strength-as-an-expression), same chain order. One new
 word.
 
+You have already seen one: the default bell flash — blinks then a breathe on
+the frame of a pane that rang, in theme's `bell` colour — is exactly this, a
+chrome tint whose amount reads `since`. (The shimmer that crosses the pane's
+body at the same moment is the same idea without the new word: an ordinary
+content pass in the same chain.) Writing your own `bell { … }` replaces both;
+`bell_indicator false` silences them with the mark.
+
 ## Two rules
 
 **`channel="fg"`, almost always.** A border's *background* is the terminal's own
diff --git a/docs/config.md b/docs/config.md
index c9578ac..686ef0e 100644
--- a/docs/config.md
+++ b/docs/config.md
@@ -182,8 +182,15 @@ dim_unfocused 60          // the one knob most people want
 states {                  // ...and the whole table underneath it
     dead { grayscale amount=200; dim amount=90 }
     suspended { grayscale amount=170; dim amount=60 }
-    scrolled { tint amount=22 color="#ff5fd7" }
+    scrolled { tint amount=22 color="#7aa2f7" }   // follows theme's scroll_bg;
+    // plus derived edge fades: the top and bottom rows melt towards an edge
+    // while there is more content past it (`above` / `below` in expressions),
+    // and turn solid at the end of the buffer -- nothing more to scroll to
     unfocused { dim amount=60 }    // writing this replaces dim_unfocused
+    // bell: a shimmer across the body (a soft sheen, gone within a second)
+    // and a chrome pass on the frame -- blinks, then a breathe -- in theme's
+    // bell colour, until the pane is looked at. See the chrome shaders page;
+    // an empty bell { } removes it, bell_indicator false silences bell + mark.
 }
 ```
 
diff --git a/docs/shaders.md b/docs/shaders.md
index 860ad24..b3c1cc1 100644
--- a/docs/shaders.md
+++ b/docs/shaders.md
@@ -12,7 +12,7 @@ touched, because rewriting text would desync selection and copy.
 ```kdl
 shaders {                       // every pane gets these, in the order written
     vignette amount=70
-    ruler amount=60 at=80 color="#ff5fd7"
+    ruler amount=60 at=80 color="#7aa2f7"
 }
 ```
 
@@ -47,7 +47,7 @@ That is how you get an effect nobody built in, without building anything:
 shaders {
     dim  amount="(y % 2) * 40"                        // scanlines
     dim  amount="(x > cols - 10) * 120"               // a right margin
-    tint amount="255 - dist(x, y, curx, cury) * 12" color="#ff5fd7"
+    tint amount="255 - dist(x, y, curx, cury) * 12" color="#7aa2f7"
 }
 ```
 
@@ -59,6 +59,7 @@ shaders {
 | | `focused` — 0 or 1 |
 | | `t` — milliseconds, for animation |
 | | `since` — milliseconds the pane has been in its current state |
+| | `above` `below` — lines of scrollback hidden past the viewport's top and bottom edges, both 0 at the present |
 | operators | `+ - * / %` (integer; division by zero is 0, not a crash) |
 | | `< > <= >= == != && \|\| !` give 0 or 1, so `(x < 10) * 200` is a rule |
 | | `& \| ^ ~ << >>` on the 32-bit value, binding **tighter** than comparison — so `x & 7 == 0` asks what it looks like, unlike C |
@@ -105,7 +106,7 @@ Edit, save, look, guess again is a slow way to arrive at a colour. A program can
 set the chains for the pane it is running in, in the same syntax the config uses:
 
 ```bash
-printf '\033]5577;1;shader;chrome;tint color="#ff5fd7" amount="abs(t / 8 %% 510 - 255)"\033\\'
+printf '\033]5577;1;shader;chrome;tint color="#7aa2f7" amount="abs(t / 8 %% 510 - 255)"\033\\'
 printf '\033]5577;1;shader;content;dim amount=90\033\\'
 printf '\033]5577;1;shader;chrome;\033\\'   # that rect: back to normal
 printf '\033]5577;1;shader;\033\\'          # no rect named, so both of them