shaders: -Dprebuilt-shaders, so a gui build needs no Vulkan SDK

glslc is the one build input that wants a tool a stock machine does not have,
and it is also the input that changes least often: eight GLSL files that have
outlived several rewrites of everything around them. Asking every machine that
wants to run the SDL shell for shaderc is the wrong trade.

The SPIR-V is now COMMITTED, under shaders/prebuilt/, and -Dprebuilt-shaders
embeds that copy instead of shelling out. The default stays the honest one --
compile the shaders that are actually in the tree -- because the flag trades a
dependency for a freshness problem: with it on, the .glsl sources are not build
inputs at all, so editing one changes nothing.

`zig build shaders` is the other half, and it is deliberately independent of
-Dplatform: it recompiles every shader and writes the result back into the
tracked directory, so whoever changes a shader refreshes the cache on a machine
that has the compiler and commits the diff. `jj diff shaders/prebuilt` after it
is the freshness check -- empty means the cache was already current.

The shader list is also spelled once now (gui_shaders): the eight embeds, the
eight glslc runs and the refresh step all read it, so adding a shader is a name
there plus the @embedFile in gui.zig, not three edits in two places.

Verified: -Dplatform=gui -Dprebuilt-shaders builds with glslc absent from PATH,
and image-harness passes on that binary -- real SDL GPU pipelines built from the
committed SPIR-V, 512 source pixels read back. The default gui build still runs
the eight glslc steps; tty runs none. The committed bytes are identical to a
fresh glslc run, and `zig build shaders` is idempotent.

1 files changed, 10 insertions, 10 deletions

diff --git a/test/snapshots/topbarexec.golden b/test/snapshots/topbarexec.golden
index bed96fbd..8d15d6c4 100644
--- a/test/snapshots/topbarexec.golden
+++ b/test/snapshots/topbarexec.golden
@@ -21,16 +21,16 @@
 |
 == snap noarg-runs grid=90x20 cursor=14,3
 |New Newcol Find Grep Help Tutor Dump NextColor Debug Kill
-|  /tmp/pardes-snap/topbarexec/cwd New Del      /tmp/pardes-s╭────────────────────────────╮
-|  ls                                           ls           │pardes  resizes=1           │
-|  zmarkfile.txt                                zmarkfile.txt│terms 3  cols 2             │
-|                                                            │active #0  43x9             │
-|                                                            │scroll 0/9                  │
-|                                                            │pinch  1.0000               │
-|                                                            │tscroll  0.0000             │
-|                                                            ╰────────────────────────────╯
-|
-|
+|  /tmp/pardes-snap/topbarexec/cwd New Del      /tmp/╭────────────────────────────────────╮
+|  ls                                           ls   │pardes  resizes=1                   │
+|  zmarkfile.txt                                zmark│theme  helix                        │
+|                                                    │font   default                      │
+|                                                    │terms 3  cols 2                     │
+|                                                    │active #0  43x9                     │
+|                                                    │scroll 0/9                          │
+|                                                    │pinch  1.0000                       │
+|                                                    │tscroll  0.0000                     │
+|                                                    ╰────────────────────────────────────╯
 |  /tmp/pardes-snap/topbarexec/cwd New Del
 |  ls
 |  zmarkfile.txt