fix(test_mesh): Use uniform floor scale to fix shadow artifacts

Fixed floor shadow stretching caused by extreme non-uniform scaling.

ROOT CAUSE:
Floor plane used scale(20.0, 0.01, 20.0) - a 2000:1 scale ratio!
When transforming shadow ray points into local space:
  - Y coordinates scaled by 1/0.01 = 100x
  - sdPlane distance calculation returns distorted values
  - Shadow raymarching fails, causing stretching artifacts

ISSUE:
  floor.scale = vec3(20.0f, 0.01f, 20.0f); // ❌ Extreme non-uniform scale
  // In local space: dot(p_local, (0,1,0)) + 0.0
  // But p_local.y is 100x larger than world-space distance!

FIX:
  floor.scale = vec3(1.0f, 1.0f, 1.0f); // ✓ Uniform scale
  floor.position = vec3(0, 0, 0);       // Explicit ground level

EXPLANATION:
For PLANE objects, XZ scale doesn't matter (planes are infinite).
Y scale distorts the SDF distance calculation.
Uniform scale preserves correct world-space distances.

RESULT:
- Floor shadows now render correctly
- No stretching toward center
- Shadow distances accurate for soft shadow calculations

COMBINED WITH PREVIOUS FIXES:
1. Shader normal transformation (double-transpose fix)
2. Quaternion axis normalization (rotation stretching fix)
3. Mesh shadow scaling exclusion (AABB size fix)
4. Floor uniform scale (this fix)

Task A (test_mesh visualization) now FULLY RESOLVED.

handoff(Claude): All mesh transformation and shadow bugs fixed. Meshes
rotate correctly, normals transform properly, shadows render accurately.
Remaining known limitation: mesh shadows use AABB (axis-aligned), so
they don't rotate with the mesh - this is expected AABB behavior.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/tests/test_mesh.cc b/src/tests/test_mesh.cc
index f29ec27..75b45ac 100644
--- a/src/tests/test_mesh.cc
+++ b/src/tests/test_mesh.cc
@@ -284,7 +284,8 @@ int main(int argc, char** argv) {
 
   // --- Create Scene ---
   Object3D floor(ObjectType::PLANE);
-  floor.scale = vec3(20.0f, 0.01f, 20.0f); // Very thin floor proxy
+  floor.position = vec3(0, 0, 0); // Floor at ground level
+  floor.scale = vec3(1.0f, 1.0f, 1.0f); // Uniform scale (planes are infinite anyway)
   floor.color = vec4(0.5f, 0.5f, 0.5f, 1.0f);
   g_scene.add_object(floor);