feat: don't draw the fractal each frames

src/shaders/blit.wgsl

@@ -0,0 +1,33 @@
+// Passthrough blit: sample the cached fractal texture and write it to egui's
+// target. Kept separate from the fractal shader so the expensive per-pixel
+// iteration runs only when the cache is (re)rendered, while every egui frame
+// pays just this cheap textured fullscreen triangle.
+
+@group(0) @binding(0) var cache_tex: texture_2d<f32>;
+@group(0) @binding(1) var cache_samp: sampler;
+
+struct VsOut {
+    @builtin(position) pos: vec4<f32>,
+    @location(0) uv: vec2<f32>,
+};
+
+@vertex
+fn vs_main(@builtin(vertex_index) idx: u32) -> VsOut {
+    var verts = array<vec2<f32>, 3>(
+        vec2<f32>(-1.0, -1.0),
+        vec2<f32>(3.0, -1.0),
+        vec2<f32>(-1.0, 3.0),
+    );
+    let p = verts[idx];
+    var out: VsOut;
+    out.pos = vec4<f32>(p, 0.0, 1.0);
+    // Map NDC to texture UV. v is flipped so the cache's top row (rendered at
+    // NDC y = +1) shows at the top of the screen.
+    out.uv = vec2<f32>((p.x + 1.0) * 0.5, (1.0 - p.y) * 0.5);
+    return out;
+}
+
+@fragment
+fn fs_main(in: VsOut) -> @location(0) vec4<f32> {
+    return textureSampleLevel(cache_tex, cache_samp, in.uv, 0.0);
+}