MToon fragment shader is ALU/texture heavy with no early-exit

## Summary
The MToon fragment shader (`mtoon_fragment_v2`) samples up to 8 textures per fragment, runs 3 light loops, and performs expensive transcendental math (`pow`, `cos`, `sin`, `dfdx`/`dfdy` for TBN reconstruction). There is no early-exit for simple or unlit materials.

## Impact
- High ALU and texture pressure on the GPU.
- Screen-space TBN reconstruction (`dfdx`/`dfdy` + `rsqrt`) is executed for every normal-mapped fragment.
- `pow()` for rim Fresnel is evaluated per fragment even when rim lighting contribution is near zero.

## Location
- Fragment shader: `Sources/VRMMetalKit/Shaders/MToonShader.metal:514–708`
- TBN reconstruction: `:563–576`
- Rim pow: `:699`, `:731`
- Light loops: `:612–636`

## Suggested Fixes
1. **Shader specialization via `[[function_constant]]`:** Compile variants without normal maps, without rim, or with 0/1/2 lights.
2. **Early-exit branch:** Add a fast path for materials with `shadeToony == 1.0` and no normal map / no rim.
3. **Precompute TBN:** Move TBN to vertex shader if the normal map transform is uniform.
4. **Use half-precision** for fragment color math where possible (per roadmap item #7).

Start with Sources/VRMMetalKit/Shaders/MToonShader.metal:514–708, focusing on TBN reconstruction at 563–576, light loops at 612–636, and rim pow calls at 699 and 731. Compare the proposed specialization, early-exit, TBN, and precision approaches against the shader’s existing paths. Done means a selected optimization preserves MToon behavior and reduces the identified ALU or texture costs.

Written by the indexing model from the issue text.