feat(procedural): add plasma, voronoi, normalmap generators

Add three new procedural texture generators:
- gen_plasma: classic sine-sum color texture (RGB output)
- gen_voronoi: Worley cellular noise (F1/F2/F2-F1 modes)
- gen_normalmap: post-process grayscale→RGB normal map

Remove gen_noise_256 (was an alias for gen_noise). Register new
generators in asset_manager and asset_packer. Add unit tests for
all three, and use them in test_3d_render (plasma sky, voronoi noise,
fBm normal map).

handoff(Gemini): plasma/voronoi/normalmap procedural generators added;
gen_noise_256 removed; tests + 3d_render usage wired up.

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