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, 15 insertions, 0 deletions

diff --git a/doc/COMPLETED.md b/doc/COMPLETED.md
index c7b2cae..dddcc3e 100644
--- a/doc/COMPLETED.md
+++ b/doc/COMPLETED.md
@@ -29,6 +29,21 @@ Detailed historical documents have been moved to `doc/archive/` for reference:
 
 Use `read @doc/archive/FILENAME.md` to access archived documents.
 
+## Recently Completed (February 14, 2026)
+
+- [x] **Fullscreen Vertex Shader Modularization**
+    - **Goal**: Eliminate duplicate fullscreen triangle vertex shader code
+    - **Changes**:
+      - Replaced inline `vs_main()` with `#include "render/fullscreen_vs"` in 8 shaders
+      - Updated test_shader_assets.cc validation for ELLIPSE and GAUSSIAN_BLUR
+      - Modified shaders: circle_mask_compute, circle_mask_render, ellipse, gaussian_blur (main/test workspaces)
+    - **Impact**:
+      - ~60 lines eliminated across codebase
+      - Better maintainability: single source of truth for fullscreen vertex shader
+      - Consistent with existing pattern (solarize, distort, chroma_aberration, vignette)
+    - **Result**: 34/34 tests passing (100%)
+    - **Related**: Task #50 (WGSL Modularization)
+
 ## Recently Completed (February 11, 2026)
 
 - [x] **Effect Render API Refactor**