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().

1 files changed, 22 insertions, 0 deletions

diff --git a/include/png.h b/include/png.h
new file mode 100644
index 0000000..ddd1b72
--- /dev/null
+++ b/include/png.h
@@ -0,0 +1,22 @@
+/* PNG decoding for the kitty graphics protocol.
+ *
+ * libghostty-vt ships no image codec: built as a library it sets its
+ * `decode_png` hook to null, and a null hook means every `f=100` transmission
+ * is rejected outright — no image stored, no placement, nothing for the
+ * graphics layer to re-emit. A program that uploads a PNG (which is what a
+ * program with a picture on disk naturally does) therefore drew nothing at
+ * all, silently, while raw RGB worked. Installing this hook is the whole fix.
+ *
+ * PNG is the only format this can be about: the protocol has exactly three
+ * (`f=24` RGB, `f=32` RGBA, `f=100` PNG) and the first two are raw pixels the
+ * library already handles. Decoding only, never encoding — we hand the client
+ * RGBA.
+ */
+#ifndef SLOSH_PNG_H
+#define SLOSH_PNG_H
+
+/* Install the process-global PNG decoder. Must be called before any terminal
+ * exists, and is idempotent. */
+void png_init(void);
+
+#endif /* SLOSH_PNG_H */