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/specs/depth-scaled-tangent-normals/spec.md

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+# depth-scaled-tangent-normals Specification
+
+## Purpose
+TBD - created by archiving change scale-tangent-normals-with-depth. Update Purpose after archive.
+## Requirements
+### Requirement: Normal strength follows geometry depth scale
+When a Geometry Nodes group both exposes `Depth Scale` and is rendered with a normal map, it SHALL store `1 - Depth Scale` as the `o_normal_reduction` float attribute on its final geometry. `O Image Layer` SHALL calculate Tangent and Object Normal Map Strength as its `Normal Scale` input multiplied by `1 - o_normal_reduction`.
+
+#### Scenario: Reduced relief depth reduces Tangent normal strength
+- **WHEN** an O Image Relief Plane uses a Tangent normal map and `Depth Scale` is set to 0.5
+- **THEN** the Tangent Normal Map Strength is one half of the value produced at `Depth Scale` 1 with the same `Normal Scale` input
+
+#### Scenario: Zero relief depth disables Tangent normal detail
+- **WHEN** an O Image Relief Plane uses a Tangent normal map and `Depth Scale` is set to 0
+- **THEN** the Tangent Normal Map Strength is zero
+
+#### Scenario: Depth extrapolation is preserved
+- **WHEN** an eligible Tangent-normal geometry group uses `Depth Scale` greater than 1
+- **THEN** `o_normal_reduction` is negative by the corresponding amount and the Tangent Normal Map Strength retains the extrapolated value without clamping
+
+### Requirement: Reduction attribute retains its material fallback
+`O Image Layer` SHALL use a factor of 1 for its Tangent Normal Map Strength when `o_normal_reduction` is absent. The existing `o_normal_reduction` material contract SHALL remain unchanged.
+
+#### Scenario: Ordinary image material has no normal-scale attribute
+- **WHEN** an object using O Image Layer has no `o_normal_reduction` attribute
+- **THEN** its Tangent Normal Map Strength equals the `Normal Scale` input
+
+#### Scenario: Object Space normal strength follows the reduction
+- **WHEN** O Image Layer is configured for an Object Space normal map and its geometry has any `o_normal_reduction` value
+- **THEN** the Object Space Normal Map Strength equals the `Normal Scale` input multiplied by `1 - o_normal_reduction`
+