fix: flip Y in vertex shader, dynamic viewport/scissor, clean debug

- Fix: Y coordinate was inverted (1.0 - 2.0*y → 2.0*y - 1.0)
  Vulkan clip space Y is already down-up, no need to flip
- Fix: dynamic viewport/scissor with viewport_count(1) + scissor_count(1)
  Required by validation layer even with dynamic state
- Clean: removed debug eprintlns, restored default clear color
- Shaders recompiled to SPIR-V

assets/shaders/cell.vert

@@ -16,12 +16,12 @@ layout(location = 1) out vec4 out_fg;
 layout(location = 2) out vec4 out_bg;
 
 void main() {
-    vec2 pixel = in_grid * pc.cell_size + in_pos * pc.cell_size;
-    vec2 clip = vec2(
+    vec2 pixel = (in_grid + in_pos) * pc.cell_size;
+    gl_Position = vec4(
         2.0 * pixel.x / pc.screen_size.x - 1.0,
-        1.0 - 2.0 * pixel.y / pc.screen_size.y
+        2.0 * pixel.y / pc.screen_size.y - 1.0,
+        0.0, 1.0
     );
-    gl_Position = vec4(clip, 0.0, 1.0);
     out_uv = in_atlas.xy + in_pos * in_atlas.zw;
     out_fg = in_fg;
     out_bg = in_bg;