refactor(wgsl): replace inline fullscreen_vs with common include

Replace duplicate fullscreen triangle vertex shader code with
#include "render/fullscreen_vs" in 8 workspace shaders. Eliminates
~60 lines of duplication and establishes single source of truth.

Modified shaders:
- circle_mask_compute.wgsl (main/test)
- circle_mask_render.wgsl (main/test)
- ellipse.wgsl (main/test)
- gaussian_blur.wgsl (main/test)

Updated test_shader_assets.cc to validate include directive instead
of inline @vertex keyword for affected shaders.

All tests passing (34/34).

handoff(Claude): Shader modularization - fullscreen_vs consolidated

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 2 insertions, 10 deletions

diff --git a/workspaces/test/shaders/circle_mask_compute.wgsl b/workspaces/test/shaders/circle_mask_compute.wgsl
index 484d3dd..b167781 100644
--- a/workspaces/test/shaders/circle_mask_compute.wgsl
+++ b/workspaces/test/shaders/circle_mask_compute.wgsl
@@ -2,6 +2,8 @@
 // Generates a circular mask (1.0 inside, 0.0 outside)
 
 #include "common_uniforms"
+#include "render/fullscreen_vs"
+
 struct CircleMaskParams {
   radius: f32,
   _pad0: f32,
@@ -12,16 +14,6 @@ struct CircleMaskParams {
 @group(0) @binding(0) var<uniform> uniforms: CommonUniforms;
 @group(0) @binding(1) var<uniform> params: CircleMaskParams;
 
-struct VSOutput {
-  @builtin(position) position: vec4<f32>,
-};
-
-@vertex fn vs_main(@builtin(vertex_index) i: u32) -> VSOutput {
-  var pos = array<vec2<f32>, 3>(
-    vec2<f32>(-1, -1), vec2<f32>(3, -1), vec2<f32>(-1, 3));
-  return VSOutput(vec4<f32>(pos[i], 0.0, 1.0));
-}
-
 @fragment fn fs_main(@builtin(position) p: vec4<f32>) -> @location(0) vec4<f32> {
   let uv = p.xy / uniforms.resolution;
   let center = vec2<f32>(0.5, 0.5);