fix: negate Y in perspective() to correct rasterized 3D orientation

The fullscreen post-process VS uses Y-up UVs (uv.y=0 = bottom), so
textureSample() Y-flips any rasterized offscreen texture. SDF effects
author their content Y-down and look correct after the flip. Rasterized
effects (RotatingCube, Hybrid3D) must pre-flip their geometry:

- mat4::perspective(): m[5] = -t (negated Y scale)
- Pipelines with cullMode=Back: frontFace = WGPUFrontFace_CW (Y-flip
  reverses winding, so CW becomes the visible face)
- Remove incorrect transposes from GlobalUniforms::make(),
  ObjectData::make(), and Uniforms::make() — mini_math is column-major,
  no transpose needed for GPU upload
- Document the convention in doc/3D.md under "Rasterized 3D and the
  Y-flip rule"

handoff(Gemini): Y-flip rule now documented; all rasterized 3D pipelines
must follow it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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