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-compact-repeat-node-arrangement/proposal.md

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+## Why
+
+Repeat 为取值支线预留宽度，导致单个 Set Position 的循环被拉长，前置循环的处理顺序也难以辨认。
+
+## What Changes
+
+- 按循环执行主链排列 Repeat 两端，取值支线独立排列。
+- 几何处理节点按真实上游依赖确定先后位置。
+- 更新排列测试、节点资产和交互预览。
+- 将长计算链紧凑排列，为复杂支线添加 Frame，并分离几何引用线通道。
+
+## Capabilities
+
+### New Capabilities
+
+- `compact-repeat-arrangement`: 紧凑呈现循环执行主链并保持几何处理顺序。
+
+### Modified Capabilities
+
+## Impact
+
+涉及 `tools/nodes/arrangement`、`tools/nodes/preview`、相关测试和 `O_AnyImage.blend`。