Add parallax occlusion mapping to PBR materials

[SpectralDragon]

Summary

• Add PBRMaterial.heightTexture, heightScale (world units), heightTextureCoordinate, heightMaxLayers and heightFadeDistance.
• Ray-march the height map in flat3d.glsl; the UV shift comes from screen-space derivatives, so no tangents are needed, and it applies to every material texture sharing the height map's UV set.
• The height map is read with the base color sampler, because Metal has no free sampler slot (0–15 are taken).
• Depth, shadows and silhouettes are not displaced. There is no glTF import yet because no displacement extension is ratified.
• Record the change in ADR-0027 (Stage 2).

Depth of field and color grading were already implemented (ADR-0027 Stage 1), so this PR only adds parallax.

Test plan

• Render3DParallaxSettingsTests (uniform packing, clamping, revision invalidation)
• Render3DGPUTests/Parallax on Metal (zero scale is identical to flat; the raised bar extends toward the camera)
• All Render3DGPUTests on Metal (35 tests, golden frames unchanged)
• MaterialUploadReuseTests, PBR3DSceneTests
• WebGPU (script/validate_webgpu.sh) and Windows, not run (the shader uses textureGrad)

🤖 Generated with Claude Code

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-18T22:45:30.330602Z	b86b2fa	Draft marked ready

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