fix(shaders): deduplicate VertexOutput/vs_main via render/fullscreen_uv_vs snippet

- Fix vs_main return type (VertexOutput, not vec4<f32>)
- Fix #include paths in passthrough, gaussian_blur, heptagon, combined_postprocess
- ShaderComposer: assert + suggest correct path on missing #include (non-STRIP_ALL)
- VerifyIncludes: upgrade WARNING to ERROR + assert, add "did you mean?" hint

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

6 files changed, 20 insertions, 75 deletions

diff --git a/common/shaders/combined_postprocess.wgsl b/common/shaders/combined_postprocess.wgsl
index d56386e..bf43d6e 100644
--- a/common/shaders/combined_postprocess.wgsl
+++ b/common/shaders/combined_postprocess.wgsl
@@ -3,25 +3,12 @@
 
 #include "sequence_uniforms"
 #include "postprocess_inline"
+#include "render/fullscreen_uv_vs"  // <- VertexOutput + vs_main
 
 @group(0) @binding(0) var input_sampler: sampler;
 @group(0) @binding(1) var input_texture: texture_2d<f32>;
 @group(0) @binding(2) var<uniform> uniforms: UniformsSequenceParams;
 
-struct VertexOutput {
-    @builtin(position) position: vec4<f32>,
-    @location(0) uv: vec2<f32>,
-};
-
-@vertex fn vs_main(@builtin(vertex_index) vid: u32) -> VertexOutput {
-    var out: VertexOutput;
-    let x = f32((vid & 1u) << 1u);
-    let y = f32((vid & 2u));
-    out.position = vec4<f32>(x * 2.0 - 1.0, 1.0 - y * 2.0, 0.0, 1.0);
-    out.uv = vec2<f32>(x, y);
-    return out;
-}
-
 @fragment fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
     // Sample base color
     var color = textureSample(input_texture, input_sampler, in.uv);
diff --git a/common/shaders/gaussian_blur.wgsl b/common/shaders/gaussian_blur.wgsl
index 293977f..551d522 100644
--- a/common/shaders/gaussian_blur.wgsl
+++ b/common/shaders/gaussian_blur.wgsl
@@ -1,5 +1,6 @@
 // Gaussian blur shader for Sequence v2
 #include "sequence_uniforms"
+#include "render/fullscreen_uv_vs"  // <- VertexOutput + vs_main
 
 @group(0) @binding(0) var input_sampler: sampler;
 @group(0) @binding(1) var input_texture: texture_2d<f32>;
@@ -12,20 +13,6 @@ struct GaussianBlurParams {
 };
 @group(0) @binding(3) var<uniform> params: GaussianBlurParams;
 
-struct VertexOutput {
-    @builtin(position) position: vec4<f32>,
-    @location(0) uv: vec2<f32>,
-};
-
-@vertex fn vs_main(@builtin(vertex_index) vid: u32) -> VertexOutput {
-    var out: VertexOutput;
-    let x = f32((vid & 1u) << 1u);
-    let y = f32((vid & 2u));
-    out.position = vec4<f32>(x * 2.0 - 1.0, 1.0 - y * 2.0, 0.0, 1.0);
-    out.uv = vec2<f32>(x, y);
-    return out;
-}
-
 @fragment fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
     let texel_size = 1.0 / uniforms.resolution;
     let offset = params.direction * texel_size;
diff --git a/common/shaders/heptagon.wgsl b/common/shaders/heptagon.wgsl
index 078978a..519dce5 100644
--- a/common/shaders/heptagon.wgsl
+++ b/common/shaders/heptagon.wgsl
@@ -1,23 +1,10 @@
 // Heptagon shader for Sequence v2
 #include "sequence_uniforms"
+#include "render/fullscreen_uv_vs"  // <- VertexOutput + vs_main
 
 // Standard v2 post-process layout (bindings 0,1 unused for scene effects)
 @group(0) @binding(2) var<uniform> uniforms: UniformsSequenceParams;
 
-struct VertexOutput {
-    @builtin(position) position: vec4<f32>,
-    @location(0) uv: vec2<f32>,
-};
-
-@vertex fn vs_main(@builtin(vertex_index) vid: u32) -> VertexOutput {
-    var out: VertexOutput;
-    let x = f32((vid & 1u) << 1u);
-    let y = f32((vid & 2u));
-    out.position = vec4<f32>(x * 2.0 - 1.0, 1.0 - y * 2.0, 0.0, 1.0);
-    out.uv = vec2<f32>(x, y);
-    return out;
-}
-
 fn sdf_heptagon(p: vec2<f32>, r: f32) -> f32 {
     let an = 3.141593 / 7.0;  // PI/7 for heptagon
     let acs = vec2<f32>(cos(an), sin(an));
diff --git a/common/shaders/passthrough.wgsl b/common/shaders/passthrough.wgsl
index 9fb0bdc..265082a 100644
--- a/common/shaders/passthrough.wgsl
+++ b/common/shaders/passthrough.wgsl
@@ -1,24 +1,11 @@
 // Passthrough shader for Sequence v2
 #include "sequence_uniforms"
+#include "render/fullscreen_uv_vs"  // <- VertexOutput + vs_main
 
 @group(0) @binding(0) var input_sampler: sampler;
 @group(0) @binding(1) var input_texture: texture_2d<f32>;
 @group(0) @binding(2) var<uniform> uniforms: UniformsSequenceParams;
 
-struct VertexOutput {
-    @builtin(position) position: vec4<f32>,
-    @location(0) uv: vec2<f32>,
-};
-
-@vertex fn vs_main(@builtin(vertex_index) vid: u32) -> VertexOutput {
-    var out: VertexOutput;
-    let x = f32((vid & 1u) << 1u);
-    let y = f32((vid & 2u));
-    out.position = vec4<f32>(x * 2.0 - 1.0, 1.0 - y * 2.0, 0.0, 1.0);
-    out.uv = vec2<f32>(x, y);
-    return out;
-}
-
 @fragment fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
     return textureSample(input_texture, input_sampler, in.uv);
 }
diff --git a/common/shaders/render/fullscreen_uv_vs.wgsl b/common/shaders/render/fullscreen_uv_vs.wgsl
new file mode 100644
index 0000000..42d87c3
--- /dev/null
+++ b/common/shaders/render/fullscreen_uv_vs.wgsl
@@ -0,0 +1,15 @@
+// Common vertex shader for fullscreen post-processing effects.
+// Draws a single triangle that covers the entire screen, with uv
+// coordinates in [0..1]
+struct VertexOutput {
+    @builtin(position) position: vec4<f32>,
+    @location(0) uv: vec2<f32>,
+};
+
+@vertex fn vs_main(@builtin(vertex_index) i: u32) -> VertexOutput {
+    let pos = array<vec4f, 3>(vec4f(-1, -1, 0, 1.), vec4f(3, -1, 2., 1.), vec4f(-1, 3, 0, -1));
+    var out: VertexOutput;
+    out.position = vec4f(pos[i].xy, 0.0, 1.0);
+    out.uv = pos[i].zw;
+    return out;
+}
diff --git a/common/shaders/skybox.wgsl b/common/shaders/skybox.wgsl
index 31bea3b..d280390 100644
--- a/common/shaders/skybox.wgsl
+++ b/common/shaders/skybox.wgsl
@@ -1,29 +1,11 @@
 #include "common_uniforms"
 #include "math/common_utils"
+#include "render/fullscreen_uv_vs"  // <- VertexOutput + vs_main
 
 @group(0) @binding(0) var sky_tex: texture_2d<f32>;
 @group(0) @binding(1) var sky_sampler: sampler;
 @group(0) @binding(2) var<uniform> globals: GlobalUniforms;
 
-struct VertexOutput {
-    @builtin(position) position: vec4<f32>,
-    @location(0) uv: vec2<f32>,
-};
-
-@vertex
-fn vs_main(@builtin(vertex_index) vertex_index: u32) -> VertexOutput {
-    var pos = array<vec2<f32>, 3>(
-        vec2<f32>(-1.0, -1.0),
-        vec2<f32>( 3.0, -1.0),
-        vec2<f32>(-1.0,  3.0)
-    );
-    
-    var out: VertexOutput;
-    out.position = vec4<f32>(pos[vertex_index], 0.0, 1.0);
-    out.uv = vec2<f32>(pos[vertex_index].x * 0.5 + 0.5, 1.0 - (pos[vertex_index].y * 0.5 + 0.5));
-    return out;
-}
-
 @fragment
 fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
     // Convert UV to NDC