Post chain: Shadertoy passes compiled by glslc; Crt, Ripple and Glitch rewritten as Shadertoy files

Stage 9 of docs/render-pipeline.md. src/gui/Post.zig runs the chain over
the finished frame: each pass reads the one before, ping-pong textures
between, the last writes the window; an empty chain is the direct path.
The prefix (shaders/post/prefix.glsl) is ghostty's uniform block, name
for name and offset for offset (tested against a copy of its Uniforms, the
selection colours by their GLSL names), at SDL GPU's bindings, y down.

A user's file (Shader <path>) is compiled by glslc with compileGlsl's own
flags on a thread of its own, taken in at the next loop step; errors name
the file's own lines; a failed compile keeps the last good pipeline;
glslc missing is said once. The bundled passes compile with the build,
through the same prefix.

Crt, Ripple and Glitch are Shadertoy files, rewritten: no barrel (input is
identity, tested), everything in linear light and dithered, keyed to iTime.
scene_effects, crt.zig and crt.frag.glsl are gone; the core keeps no clock
for the chain. ShaderAnimation off|on|always drives redraw level A: the
chain alone over the retained frame (34 us CPU a redraw, measured).

Shared files touched: pardes.zig (nextWake loses the scene line,
disableSceneEffects empties the chain, two tests), builtins.zig (one switch
arm), ninep/ctl.zig (two switch arms), macos.zig (flags from the chain),
gui.zig. Not touched: Messages.zig, mouse.zig, detached/*, host_io, tty.

1 files changed, 79 insertions, 0 deletions

diff --git a/shaders/post/crt.glsl b/shaders/post/crt.glsl
new file mode 100644
index 00000000..16a5cdd9
--- /dev/null
+++ b/shaders/post/crt.glsl
@@ -0,0 +1,79 @@
+// Crt: a slot-mask tube's light, with nothing that moves a pixel. There is
+// no barrel: every click lands where it is drawn, rails and grips at the
+// window's edges included. What is left is light: a beam profile for the
+// scanlines, an aperture mask, a soft bloom of the brightest ink, a faint
+// vignette and the hum of a live tube. Everything is worked out in linear
+// light and dithered back into 8 bits, so the vignette never bands. It is a
+// Shadertoy file: pardes puts its prefix before it (shaders/post/prefix.glsl).
+
+const float TAU = 6.28318530718;
+const float CHROMA_PX = 0.35;       // red and blue apart, at the corners
+const float BLOOM_RADIUS_PX = 1.75;
+const float BLOOM_THRESHOLD = 0.60; // linear-light channel peak
+const float BLOOM_SOFTNESS = 0.25;
+const float BLOOM_STRENGTH = 0.05;
+const float SCAN_PERIOD_PX = 3.0;
+const float SCAN_STRENGTH = 0.07;
+const float MASK_PERIOD_PX = 3.0;
+const float MASK_STRENGTH = 0.018;
+const float VIGNETTE_STRENGTH = 0.12; // at the far corners only
+const float HUM_STRENGTH = 0.004;
+const float HUM_HZ = 0.11;
+const float EXPOSURE = 1.015;
+
+vec3 toLinear(vec3 c) {
+    return mix(pow((c + 0.055) / 1.055, vec3(2.4)), c / 12.92, lessThanEqual(c, vec3(0.04045)));
+}
+
+vec3 toSrgb(vec3 c) {
+    c = max(c, vec3(0.0));
+    return mix(1.055 * pow(c, vec3(1.0 / 2.4)) - 0.055, c * 12.92, lessThanEqual(c, vec3(0.0031308)));
+}
+
+vec3 scene(vec2 uv) {
+    return toLinear(texture(iChannel0, uv).rgb);
+}
+
+vec3 brightPass(vec2 uv) {
+    vec3 c = scene(uv);
+    float peak = max(c.r, max(c.g, c.b));
+    return c * smoothstep(BLOOM_THRESHOLD, BLOOM_THRESHOLD + BLOOM_SOFTNESS, peak);
+}
+
+// Interleaved gradient noise: a dither with no visible pattern.
+float ign(vec2 p) {
+    return fract(52.9829189 * fract(dot(p, vec2(0.06711056, 0.00583715))));
+}
+
+void mainImage(out vec4 fragColor, in vec2 fragCoord) {
+    vec2 size = iResolution.xy;
+    vec2 texel = 1.0 / size;
+    vec2 uv = fragCoord / size;
+    vec2 cc = uv * 2.0 - 1.0;
+    float r2 = dot(cc, cc);
+
+    // The beam converges least at the corners: red and blue drift apart
+    // there, by a third of a pixel.
+    vec2 chroma = cc * (CHROMA_PX * r2 * 0.5) * texel;
+    vec3 col = vec3(scene(uv + chroma).r, scene(uv).g, scene(uv - chroma).b);
+
+    vec2 b = texel * BLOOM_RADIUS_PX;
+    vec3 bloom = brightPass(uv + vec2(b.x, 0.0)) + brightPass(uv - vec2(b.x, 0.0)) +
+        brightPass(uv + vec2(0.0, b.y)) + brightPass(uv - vec2(0.0, b.y));
+    col += bloom * (0.25 * BLOOM_STRENGTH);
+
+    // A beam is brightest on its line and falls off between lines; the
+    // mask is three phosphor stripes a triad. Both average to one, so the
+    // theme keeps its brightness.
+    float line = 0.5 + 0.5 * cos(fragCoord.y * TAU / SCAN_PERIOD_PX);
+    col *= 1.0 - SCAN_STRENGTH * (line * line - 0.375) / 0.625;
+    col *= 1.0 + MASK_STRENGTH * cos(fragCoord.x * TAU / MASK_PERIOD_PX + vec3(0.0, TAU / 3.0, 2.0 * TAU / 3.0));
+
+    float vignette = 1.0 - VIGNETTE_STRENGTH * pow(clamp(r2 * 0.5, 0.0, 1.0), 1.6);
+    float hum = 1.0 + HUM_STRENGTH * sin((uv.y * 1.35 - iTime * HUM_HZ) * TAU);
+    col *= vignette * hum * EXPOSURE;
+
+    vec3 srgb = toSrgb(col);
+    srgb += (ign(fragCoord + float(iFrame % 64) * vec2(5.588238, 5.588238)) - 0.5) / 255.0;
+    fragColor = vec4(clamp(srgb, 0.0, 1.0), texture(iChannel0, uv).a);
+}