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/proposal.md

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+## Why
+
+Tangent-space normal maps describe the full-depth surface.  When a geometry node reduces its `Depth Scale`, its shape becomes flatter but the material normal remains at full strength, making surface detail appear too strong.
+
+## What Changes
+
+- Reuse the existing `o_normal_reduction` geometry attribute as the material-to-geometry contract for tangent-space normal strength.
+- Make tangent-normal depth geometry write `1 - Depth Scale` to that attribute, so the material normal strength follows the displayed geometric depth.
+- Extend the existing `O Image Layer` behavior without changing its attribute lookup or fallback semantics.
+- Make Object Space normal strength follow the same geometry reduction contract.
+- Reuse the existing `o_normal_reduction` contract for additional Tangent-normal depth geometry.
+
+## Capabilities
+
+### New Capabilities
+- `depth-scaled-tangent-normals`: Keep tangent-space material normal strength consistent with geometry depth scaling.
+
+### Modified Capabilities
+
+None.
+
+## Impact
+
+- Tangent-normal geometry-group builders, especially Relief Plane.
+- Normal-scale node tests; saved node assets must be rebuilt with `uv run node-group build`.
+- No server inference format, user-facing modifier socket, or dependency changes.