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, 14 insertions, 0 deletions

diff --git a/src/shaders/math/utils.wgsl b/src/shaders/math/utils.wgsl
new file mode 100644
index 0000000..c75cb66
--- /dev/null
+++ b/src/shaders/math/utils.wgsl
@@ -0,0 +1,14 @@
+// General-purpose math utility functions.
+
+// Returns a 2x2 rotation matrix.
+fn rot(a: f32) -> mat2x2f {
+  let c = cos(a);
+  let s = sin(a);
+  return mat2x2f(c, s, -s, c);
+}
+
+// Fast approximation of tanh.
+fn tanh_approx(x: f32) -> f32 {
+  let x2 = x * x;
+  return clamp(x * (27.0 + x2) / (27.0 + 9.0 * x2), -1.0, 1.0);
+}