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>

1 files changed, 17 insertions, 0 deletions

diff --git a/doc/3D.md b/doc/3D.md
index c27b034..dbcd6f9 100644
--- a/doc/3D.md
+++ b/doc/3D.md
@@ -24,6 +24,23 @@
 - WebGPU framebuffer: y-down (pixel row 0 at top)
 - `uv.y = 0` → NDC `y = -1` → bottom of framebuffer ✓
 
+### Rasterized 3D and the Y-flip rule
+
+The fullscreen post-process VS (`fullscreen_uv_vs.wgsl`) uses **Y-up UVs** (`uv.y=0` = bottom of screen).
+`textureSample(tex, s, uv)` with those UVs samples the **top** of the input texture at the bottom of
+the screen — a Y-flip. SDF effects are authored Y-down and appear correct after this flip. Rasterized
+effects (RotatingCube, Hybrid3D) render into an offscreen texture and are consumed by the post-process
+chain, so they must **pre-flip** their geometry to compensate:
+
+- **`mat4::perspective()`** uses `m[5] = -t` (negated Y scale) — renders geometry upside-down into the
+  texture so the post-process chain flips it right-side up.
+- **Rasterized pipelines** with `cullMode = Back` must set `frontFace = WGPUFrontFace_CW` — the Y-flip
+  reverses winding order on screen, so CW becomes the visible (front) face.
+- Pipelines with `cullMode = None` (SDF proxy hull, skybox) need no winding change.
+
+**Do not** set `m[5] = +t` or use `frontFace = WGPUFrontFace_CCW` (default) in rasterized 3D
+pipelines — the result will be upside-down with back-faces culled.
+
 ## Core Concept
 
 Hybrid SDF/rasterization pipeline with physics and collision detection.