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, 2 insertions, 33 deletions

diff --git a/test/snapshots/fsearch.golden b/test/snapshots/fsearch.golden
index bbe7df83..317e7b5e 100644
--- a/test/snapshots/fsearch.golden
+++ b/test/snapshots/fsearch.golden
@@ -60,7 +60,7 @@
 |
 |
 |   /MARK
-== snap results grid=180x30 cursor=7,2
+== snap results grid=180x30 cursor=10,3
 |New Newcol Find Grep Help Tutor Dump NextColor Debug Kill
 |  /tmp/pardes-snap/fsearch/cwd/g.txt Save New Del             /tmp/pardes-snap/fsearch/cwd   New Del                      /tmp/pardes-snap/fsearch/cwd   New Del
 |     1 l1                                                     ls                                                          ls
@@ -91,38 +91,7 @@
 |     1 g.txt:2:1-4 MARK a
 |     2 g.txt:6:1-4 MARK b
 |     3 g.txt:10:1-4 MARK c
-== snap first grid=180x30 cursor=10,3
-|New Newcol Find Grep Help Tutor Dump NextColor Debug Kill
-|  /tmp/pardes-snap/fsearch/cwd/g.txt Save New Del             /tmp/pardes-snap/fsearch/cwd   New Del                      /tmp/pardes-snap/fsearch/cwd   New Del
-|     1 l1                                                     ls                                                          ls
-|     2 MARK a                                                 g.txt                                                       g.txt
-|     3 l3
-|     4 l4
-|     5 l5
-|     6 MARK b                                                   g.txt
-|     7 l7
-|     8 l8
-|     9 l9
-|    10 MARK c
-|    11 l11
-|    12 l12
-|
-|
-|                                                              /tmp/pardes-snap/fsearch/cwd   New Del
-|                                                              ls
-|                                                              g.txt
-|
-|
-|
-|
-|
-|
-|
-|  /tmp/pardes-snap/fsearch/cwd/+Search    New Del
-|     1 g.txt:2:1-4 MARK a
-|     2 g.txt:6:1-4 MARK b
-|     3 g.txt:10:1-4 MARK c
-== style first
+== style results
 |0: 0-56 #949494,#121212, 57-179 d,#121212,
 |1: 0-1 #ffffff,#626262, 2-48 #949494,#121212, 49-59 d,#121212, 60-61 #ffffff,#262626, 62-99 #949494,#121212, 100-119 d,#121212, 120-121 #ffffff,#262626, 122-159 #949494,#121212, 160-179 d,#121212,
 |2: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-8 #bdbdbd,#080808, 9-59 d,#080808, 60-60 d,#444444, 61-61 d,#080808, 62-119 #bdbdbd,#080808, 120-120 d,#444444, 121-121 d,#080808, 122-179 #bdbdbd,#080808,