Add specifications for two-times upscale and unified depth plane features

- Introduced the two-times upscale specification detailing requirements for unified output, fixed model presets, arbitrary dimensions, and consistent job behavior.
- Added scenarios for existing models, repeated upscale, and handling of alpha preservation.
- Created the unified depth plane specification outlining geometry modes, stable interface, and asset evaluation.
- Defined scenarios for mode switching, relief and camera preservation, and validation of delivered assets.

openspec/changes/archive/2026-09-17-scale-tangent-normals-with-depth/tasks.md

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+## 1. Reuse the geometry-to-material contract
+
+- [x] 1.1 Update O Image Relief Plane to store `1 - Depth Scale` as `o_normal_reduction` on final geometry.
+- [x] 1.2 Confirm O Image Layer's existing `1 - o_normal_reduction` Tangent and Object-strength paths and its absent-attribute fallback need no code change.
+- [x] 1.3 Confirm existing O Image Cutout and O Image Depth Cutout attribute uses retain their current geometry and material behavior.
+
+## 2. Verification and assets
+
+- [x] 2.1 Update definition and rendered node tests for absent, zero, fractional, and greater-than-one `o_normal_reduction` values in both normal spaces.
+- [x] 2.2 Run the relevant node tests and fix regressions.
+- [x] 2.3 Run `uv run node-group build` and verify the regenerated `O_AnyImage.blend` asset and inventory tests.