refactor: move common/shaders/ to src/shaders/

Relocates shared WGSL shaders under src/ where all source code lives,
eliminating the top-level common/ directory.

- Update asset references in workspaces/main/assets.txt and workspaces/test/assets.txt
- Update docs: PROJECT_CONTEXT.md, ARCHITECTURE.md, WORKSPACE_SYSTEM.md, SHADER_REUSE_INVESTIGATION.md

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

1 files changed, 10 insertions, 0 deletions

diff --git a/src/shaders/render/fullscreen_vs.wgsl b/src/shaders/render/fullscreen_vs.wgsl
new file mode 100644
index 0000000..507b892
--- /dev/null
+++ b/src/shaders/render/fullscreen_vs.wgsl
@@ -0,0 +1,10 @@
+// Common vertex shader for fullscreen post-processing effects.
+// Draws a single triangle that covers the entire screen.
+@vertex fn vs_main(@builtin(vertex_index) i: u32) -> @builtin(position) vec4f {
+    var pos = array<vec2f, 3>(
+        vec2f(-1, -1),
+        vec2f( 3, -1),
+        vec2f(-1,  3)
+    );
+    return vec4f(pos[i], 0.0, 1.0);
+}