docs: Update Task A status - artifacts fixed but shadows missing

Task A progress:
✅ Mesh normal transformation fixed (shader double-transpose bug)
✅ Quaternion rotation stretching fixed (axis normalization)
✅ Mesh shadow scaling fixed (excluded from SDF scale factor)
✅ Floor rendering artifacts fixed (PLANE → BOX workaround)

Remaining issues:
- Task A.1: Missing shadows in test_mesh (needs investigation)
- Task A.2: ObjectType::PLANE broken with non-uniform scaling
  - Root cause: sdPlane doesn't handle non-uniform scale in local space
  - Workaround applied: Using BOX instead of PLANE for floor
  - Need to determine if PLANE should support this or is unsupported by design

handoff(Claude): Test mesh visual artifacts resolved, but PLANE object type
has fundamental issues with non-uniform scaling. Workaround in place, but
proper fix requires deeper investigation into SDF distance field math.

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