feat: procedural terrain generators with example script and bilingual tutorial docs

[wlgys8]

Summary

• 新增程序化高度场地形生成器（motrix_env_core/config/scene/terrain.py）：Stairs（线性/径向，四种 profile、固定踏面、中心平台、base_level）、DiscreteObstacles（fixed/choice）、PyramidSlope（含反转）、Quantized 台地化，以及组合地形 CompositeTerrainGeneratorCfg / TerrainRegionCfg / grid_terrain；物理高度在组合时守恒，validate() 拒绝超出 height_scale 的配置
• 新增示例脚本 examples/terrain_generate.py：展示每种生成器与组合布局，打印高度统计、保存灰度高度图，--render 打开查看器
• 新增用户文档「程序化地形生成」（中英双语 docs/source/{zh_CN,en}/user_guide/tutorial/building_envs/terrain.md），加入两侧 toctree 与阅读路径；覆盖生成器契约、各生成器字段、组合地形、环境中替换 scene 的模式、sample_terrain_height 运行时采样、自定义生成器
• 10 张离屏渲染截图（docs/source/_static/images/tutorial/terrain/，JPEG 共约 640 KB）

Validation

• python -m pytest：地形相关 31 个用例通过
• prek run --all-files 全部 hook 通过
• 中英两次 sphinx-build -W --keep-going 严格构建通过；构建产物中 10 张插图与图注逐一确认
• 文档数值声明（height_scale、校验上限、字段语义）对照实现逐项复核，中英一致

[wlgys8]

组合地形（CompositeTerrainGeneratorCfg：噪声台地基底 + 径向台阶、反转金字塔坑、离散障碍物、金字塔坡）效果图：

[Copilot]

🟡 Changes recommended

The new terrain generator implementations include several correctness issues (division-by-zero/NaN cases and a duplicated __all__ export) that should be addressed before merging.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

This PR adds a new bilingual (zh_CN/en) framework tutorial on build-time procedural height-field terrain generation, along with rendered screenshots, and introduces/extends the underlying terrain generator configs in motrix_env_core.config.scene (plus tests and a preview script) to support the documented workflows.

Changes:

• Added a new motrix_env_core.config.scene.terrain module implementing multiple procedural terrain generators plus composition utilities (CompositeTerrainGeneratorCfg, TerrainRegionCfg, grid_terrain, etc.).
• Extended NoiseTerrainGeneratorCfg with optional coarse sampling + bilinear interpolation (downsampled_scale) and added an end-to-end generator showcase script (examples/terrain_generate.py).
• Added comprehensive generator contract/behavior tests and the bilingual tutorial pages + LFS-backed tutorial screenshots.

File summaries

File	Description
motrix_envs/tests/test_scene_cfg.py	Adds unit tests for generator contracts, validation, determinism, and composition behavior.
motrix_env_core/src/motrix_env_core/config/scene/terrain.py	Introduces new build-time procedural terrain generator configs and composition helpers.
motrix_env_core/src/motrix_env_core/config/scene/asset.py	Extends NoiseTerrainGeneratorCfg with downsampled_scale and bilinear resizing helper.
motrix_env_core/src/motrix_env_core/config/scene/init.py	Re-exports the new terrain generator symbols from motrix_env_core.config.scene.
examples/terrain_generate.py	Adds a runnable preview + optional MotrixSim render path for generator outputs.
docs/source/zh_CN/user_guide/tutorial/building_envs/terrain.md	New zh_CN tutorial page describing the generator contract and built-in generators with screenshots.
docs/source/zh_CN/user_guide/tutorial/building_envs/index.md	Adds the new terrain tutorial page to the zh_CN tutorial navigation/toctree.
docs/source/en/user_guide/tutorial/building_envs/terrain.md	New English tutorial page mirroring zh_CN content with screenshots.
docs/source/en/user_guide/tutorial/building_envs/index.md	Adds the new terrain tutorial page to the English tutorial navigation/toctree.
docs/source/_static/images/tutorial/terrain/stairs_pyramid.jpg	Adds LFS-backed tutorial screenshot asset.
docs/source/_static/images/tutorial/terrain/stairs_inverted_pyramid.jpg	Adds LFS-backed tutorial screenshot asset.
docs/source/_static/images/tutorial/terrain/stairs_ascending.jpg	Adds LFS-backed tutorial screenshot asset.
docs/source/_static/images/tutorial/terrain/slope_compare.jpg	Adds LFS-backed tutorial screenshot asset.
docs/source/_static/images/tutorial/terrain/obstacles_choice.jpg	Adds LFS-backed tutorial screenshot asset.
docs/source/_static/images/tutorial/terrain/noise_terraced.jpg	Adds LFS-backed tutorial screenshot asset.
docs/source/_static/images/tutorial/terrain/noise_smooth.jpg	Adds LFS-backed tutorial screenshot asset.
docs/source/_static/images/tutorial/terrain/noise_rough.jpg	Adds LFS-backed tutorial screenshot asset.
docs/source/_static/images/tutorial/terrain/grid.jpg	Adds LFS-backed tutorial screenshot asset.
docs/source/_static/images/tutorial/terrain/composite.jpg	Adds LFS-backed tutorial screenshot asset.

Review details

Suppressed comments (1)

motrix_env_core/src/motrix_env_core/config/scene/terrain.py:186

• _apply_platform() uses min(size) to convert platform_width (meters) into a fractional mask even in linear axis modes. For rectangular terrains this makes the platform band width incorrect; use the same axis-aligned extent as in generate() for linear modes.

        platform_half_frac = (self.platform_width / 2.0) / min(size)
        if self.axis == "radial":

• Files reviewed: 19/19 changed files
• Comments generated: 5
• Review effort level: Lite

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[wlgys8]

Copilot review 反馈已在 1b83034 修复：

1. 线性台阶在矩形场地上的 tread/platform 宽度：新增 _extent()——axis="x" 用 size[0]、axis="y" 用 size[1]、radial 保持 min(size)，_index_from_distance 与 _apply_platform 均改用轴向 extent。已在 12×4 m 矩形场上验证 step_width=1.0 恰好铺出 12 级、每级 1 m，y 方向恒定。
2. height_scale=0 除零/NaN（obstacles/slope/composite 三处）：在 TerrainGeneratorCfg.validate 统一要求 height_scale > 0，一处修复覆盖全部生成路径，并补充了 height_scale=0.0 的负向测试用例。
3. __all__ 重复导出 TerrainGeneratorCfg：已去重。

124 个测试通过，prek run --all-files 全部 hook 通过。