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-add-cutout-polyline/specs/mask-mode-options/spec.md

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+## ADDED Requirements
+
+### Requirement: Mask exposes Set Extend and Subtract
+
+Mask MUST 按顺序提供 `SET` / Set、`EXTEND` / Extend、`SUBTRACT` / Subtract，枚举数值分别为 0、1、2；界面和状态文本 MUST 使用 Extend。
+
+#### Scenario: User extends Alpha
+- **WHEN** 用户以 Extend 提交值为 mask 的选区到现有 alpha
+- **THEN** 结果 Alpha 为 `max(alpha, mask)`，RGB 保持不变
+
+#### Scenario: User uses Set or Subtract
+- **WHEN** 用户使用 Set 或 Subtract
+- **THEN** 结果 Alpha 分别为 `alpha * mask` 或 `alpha * (1-mask)`，RGB 保持不变
+
+### Requirement: Mask defaults to Set
+
+Scene 的 Mask Mode 和 Mask Operator 的模式属性 MUST 默认 Set，并通过同一属性定义保持一致。
+
+#### Scenario: New scene and operator defaults
+- **WHEN** 创建新 Scene 或未指定模式的 Mask Operator
+- **THEN** 模式默认值为 `SET`
+
+#### Scenario: User chooses another mode
+- **WHEN** 用户主动设置 Extend 或 Subtract 后启动 Mask
+- **THEN** 本次操作使用用户保存的模式