Shader files compile again on save; an attached GUI runs the session's post chain

shader_build.zig takes the Shadertoy compile out of the GUI's Post: the
process that holds the core compiles each chain file behind the prefix on a
thread, watches its directory (file_watch's new shader slots), and on a
change reads and hashes each file, compiling only one whose bytes moved. A
failed compile keeps the last good SPIR-V and is said once; the same bytes
are never compiled or reported twice. A file is read with std, not fs.zig's
readFile, whose turn hand-off a worker thread does not hold.

A detached session compiles for its frontends and sends them the chain
(wire post: each pass's scene and level or the file's SPIR-V, and
ShaderAnimation) on attach and on every change, so an attached GUI runs the
same passes, levels and animation as a local one while still reading no disk
and running no program. The attached GUI describes its frames to the chain
from the session's chrome and redraws an animating chain on its own.

1 files changed, 5 insertions, 2 deletions

diff --git a/docs/config.md b/docs/config.md
index 189c5382..59883b59 100644
--- a/docs/config.md
+++ b/docs/config.md
@@ -543,8 +543,11 @@ Crt 3
 ```
 
 The SDL GUI runs it as the bundled pass of its post chain, which also takes
-Shadertoy files written for ghostty (`Shader ~/crt.glsl`, `Shader off`), and
-`ShaderAnimation off|on|always` says when the chain animates on its own. With
+Shadertoy files written for ghostty (`Shader ~/crt.glsl`, `Shader off`;
+a file compiles again when it is saved, and a save that fails to compile
+keeps the last good one and says why), and
+`ShaderAnimation off|on|always` says when the chain animates on its own. A
+GUI attached to a detached session runs the session's chain the same way. With
 the chain empty the pass is bypassed. CRT works in linear light with restrained
 scanlines, mask, bloom and vignette, and no curvature, so clicks land where
 they are drawn.