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, 9 insertions, 40 deletions

diff --git a/test/snapshots/lookrange.golden b/test/snapshots/lookrange.golden
index d1fc0bea..8ed06206 100644
--- a/test/snapshots/lookrange.golden
+++ b/test/snapshots/lookrange.golden
@@ -277,38 +277,7 @@
 |27: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-149 d,#080808,
 |28: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-149 d,#080808,
 |29: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-149 d,#080808,
-== snap results grid=150x30 cursor=10,24
-|New Newcol Find Grep Help Tutor Dump NextColor Debug Kill
-|  /tmp/pardes-snap/lookrange/cwd/look.txt                                                                             Save New Del
-|     1 r.txt:3-5
-|     2 r.txt:7:4-9
-|     3 r.txt:3:2-5:3
-|     4 r.txt:8:6-99:9
-|
-|
-|
-|
-|
-|
-|
-|
-|
-|
-|  /tmp/pardes-snap/lookrange/cwd/r.txt                                                                                Save New Del
-|     2 bbbb
-|     3 cccc dccc
-|     4 dddd
-|     5 eeee
-|     6 ffff
-|     7 gg NEEDLE gg
-|     8 hhhh NEEDLE
-|     9 iiii
-|
-|
-|  /tmp/pardes-snap/lookrange/cwd/+Search                                                                                   New Del
-|     1 r.txt:7:4-9 gg NEEDLE gg
-|     2 r.txt:8:6-11 hhhh NEEDLE
-== snap hit1 grid=150x30 cursor=15,22
+== snap hit1 grid=150x30 cursor=15,23
 |New Newcol Find Grep Help Tutor Dump NextColor Debug Kill
 |  /tmp/pardes-snap/lookrange/cwd/look.txt                                                                             Save New Del
 |     1 r.txt:3-5
@@ -326,6 +295,7 @@
 |
 |
 |  /tmp/pardes-snap/lookrange/cwd/r.txt                                                                                Save New Del
+|     1 aaaa
 |     2 bbbb
 |     3 cccc dccc
 |     4 dddd
@@ -335,7 +305,6 @@
 |     8 hhhh NEEDLE
 |     9 iiii
 |
-|
 |  /tmp/pardes-snap/lookrange/cwd/+Search                                                                                   New Del
 |     1 r.txt:7:4-9 gg NEEDLE gg
 |     2 r.txt:8:6-11 hhhh NEEDLE
@@ -357,15 +326,15 @@
 |14: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-149 d,#080808,
 |15: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-149 d,#080808,
 |16: 0-1 #ffffff,#626262, 2-129 #949494,#121212, 130-149 d,#121212,
-|17: 0-0 d,#121212, 1-1 d,#080808, 2-6 #626262,#080808, 7-10 #bdbdbd,#080808, 11-149 d,#080808,
-|18: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-15 #bdbdbd,#080808, 16-149 d,#080808,
-|19: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-10 #bdbdbd,#080808, 11-149 d,#080808,
+|17: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-10 #bdbdbd,#080808, 11-149 d,#080808,
+|18: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-10 #bdbdbd,#080808, 11-149 d,#080808,
+|19: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-15 #bdbdbd,#080808, 16-149 d,#080808,
 |20: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-10 #bdbdbd,#080808, 11-149 d,#080808,
 |21: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-10 #bdbdbd,#080808, 11-149 d,#080808,
-|22: 0-0 d,#444444, 1-1 d,#080808, 2-6 #949494,#121212, 7-9 #bdbdbd,#080808, 10-15 #080808,#80a0ff, 16-18 #bdbdbd,#080808, 19-149 d,#080808,
-|23: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-17 #bdbdbd,#080808, 18-149 d,#080808,
-|24: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-10 #bdbdbd,#080808, 11-149 d,#080808,
-|25: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-149 d,#080808,
+|22: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-10 #bdbdbd,#080808, 11-149 d,#080808,
+|23: 0-0 d,#444444, 1-1 d,#080808, 2-6 #949494,#121212, 7-9 #bdbdbd,#080808, 10-15 #080808,#80a0ff, 16-18 #bdbdbd,#080808, 19-149 d,#080808,
+|24: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-17 #bdbdbd,#080808, 18-149 d,#080808,
+|25: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-10 #bdbdbd,#080808, 11-149 d,#080808,
 |26: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-149 d,#080808,
 |27: 0-1 #ffffff,#262626, 2-129 #949494,#121212, 130-149 d,#121212,
 |28: 0-0 d,#444444, 1-1 d,#080808, 2-6 #626262,#080808, 7-30 #bdbdbd,#080808, 31-149 d,#080808,