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/design.md

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+## Context
+
+`O Image Layer` currently reads `o_normal_reduction` and calculates Tangent and Object Normal Map strengths from `1 - attribute`. Its absence evaluates as zero, preserving full strength. `O Image Relief Plane` changes surface displacement with `Depth Scale` but supplies no matching material attribute, so its normal map remains visually over-strong at lower values.
+
+The current Depth Plane and Depth Cutout conversion paths request Object Space normals. The panorama material is shadeless. Relief Plane is the only current Depth Scale geometry path that supplies a reduction attribute, and the shared material already applies that attribute in both normal spaces.
+
+## Goals / Non-Goals
+
+**Goals:**
+
+- Reuse the existing `o_normal_reduction` attribute contract for Tangent Normal strength.
+- Keep Tangent Normal strength numerically equal to the geometry's current `Depth Scale` on Relief Plane.
+- Preserve full strength for material users and geometry that do not provide the attribute.
+- Rebuild and verify the node asset and its rendered behavior.
+
+**Non-Goals:**
+
+- Alter Object Space normal-color transformation.
+- Change Depth Scale ranges, geometry projection, normal-map generation, or user-facing sockets.
+- Add a new normal-scale attribute or alter the existing material attribute lookup.
+
+## Decisions
+
+### Reuse the reduction attribute
+
+`O Image Layer` already calculates the normal multiplier as `1 - o_normal_reduction`. Relief Plane therefore writes `1 - Depth Scale` to its final geometry. This produces a normal multiplier equal to `Depth Scale`, including zero and values greater than one, without changing the established shader contract.
+
+Alternative: introduce a second scale attribute and alter the shader to use it. This duplicates an existing contract and would require migration of current geometry producers without improving the resulting calculation.
+
+### Preserve the existing missing-attribute fallback
+
+An absent float attribute evaluates as zero, and the existing `1 - o_normal_reduction` expression therefore evaluates to one. No material-node change is required to preserve ordinary image-material strength.
+
+Alternative: add an existence switch in the material. It would replicate behavior that the current expression already provides.
+
+### Apply the reduction in both normal spaces
+
+`O Image Layer` already supplies the same `Normal Scale × (1 - o_normal_reduction)` strength field to both Tangent and Object Normal Map nodes. Relief Plane will use that established shared behavior, so both normal spaces follow its displayed geometry depth.
+
+Alternative: split the material paths and limit the factor to Tangent Space. This would introduce an inconsistent interpretation of the existing shared attribute and require a separate Object Space policy.
+
+## Risks / Trade-offs
+
+- [Relief geometry writes the reduction on an inappropriate domain] → Store the field where the material evaluates it and add rendered POINT/FACE-domain coverage.
+- [Missing attributes disable unrelated normal maps] → Test the explicit missing-attribute fallback on both Tangent and Object paths.
+- [A later Tangent Depth Scale group omits the contract] → Treat `o_normal_reduction = 1 - Depth Scale` as the required output attribute whenever a Tangent-normal geometry group exposes `Depth Scale`.
+- [Object Space visual response differs from Tangent Space] → Render and compare both paths at zero, fractional, unit, and extrapolated scales.
+
+## Migration Plan
+
+1. Update relevant Tangent-normal geometry builders to write `1 - Depth Scale` as `o_normal_reduction`.
+2. Extend rendered node tests with Relief Plane depth-scale coverage.
+3. Run focused node tests, then `uv run node-group build` to regenerate and verify `src/anyimage/assets/O_AnyImage.blend`.
+4. Roll back by restoring the previous asset and source together.