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, 6 insertions, 9 deletions

diff --git a/test/snapshots/psearch.snap b/test/snapshots/psearch.snap
index 1b9eac61..beceb806 100644
--- a/test/snapshots/psearch.snap
+++ b/test/snapshots/psearch.snap
@@ -31,19 +31,16 @@ stable 400 5000
 text MARK
 stable 400 5000
 snap typing
-# Enter opens the results buffer: three @p0:LINE:COL rows, focus stays here
+# Enter opens the results buffer — three @p0:LINE:COL rows — and goes to the
+# first of them: the terminal's cursor lands on that scrollback line, on the
+# matched SPAN (cols 3..6, past the leading "x ") — not the start of the line,
+# and selected rather than merely parked on. No `n` was pressed to get here.
 key enter
 wait 10000 @p0:2:3
 stable 700 15000
 snap results
-# n selects result 1 and Enter looks it: the terminal's cursor lands on that
-# scrollback line, on the matched SPAN (cols 3..6, past the leading "x ") —
-# not the start of the line, and selected rather than merely parked on
-key n
-key enter
-stable 700 15000
-snap first
-snapstyle first
+snapstyle results
+# n walks on to the next row, Enter looks it
 key n
 key enter
 stable 700 15000