Refactor: extract setup_standard_textures helper

Eliminates repetitive Renderer3D texture initialization across 3 test files.
Creates setup_standard_textures() helper that configures standard noise
(256x256, GPU procedural) and sky (512x256, Perlin) textures.

- New: src/3d/renderer_helpers.{h,cc}
- Refactor: test_3d_physics, test_3d_render, test_mesh use helper
- Build: Added renderer_helpers.cc to CMake 3D_SOURCES
- Result: -52 lines of boilerplate across 3 files

Production effects (Hybrid3DEffect, FlashCubeEffect) use asset-based
textures and retain custom initialization.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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