The editor’s RenderSettings and skyboxes

The MCP editor keeps a scene's RenderSettings:
- a set_render_settings command (undoable and reviewable, checked against RenderSettings' property names; null resets one);
- scene_open reads them from .unity scenes and from JSON scenes ({"renderSettings", "objects"}), and scene_query shows the ones that differ from the defaults;
- play mode applies them before Awake;
- scene_save patches only the changed fields of the .unity file's RenderSettings document, or writes the JSON object form;
- sessions handed to a new build keep them.
Skyboxes:
- Skybox/Procedural uses our own sky model with Unity's parameters: tint, ground, exposure, atmosphere thickness, and a simple or high-quality sun disk following the main light;
- Skybox/Panoramic (latitude-longitude, Unity's orientation) and Skybox/6 Sided, both with tint, exposure and rotation;
- cameras clearing to the skybox draw it behind everything, or their background color without one;
- in the skybox ambient mode, the ambient light (trilight, from cosine-weighted sky averages) and reflections come from the sky, integrated on the CPU and cached.
RenderSettings and skybox behavior:
- RenderSettings.skybox is Default-Skybox in scenes from files, and none in scenes made with CreateScene (as Unity records);
- a skybox material's edits show at once;
- a scene's "skybox" is "Default-Skybox", "None", an asset, or an inline material;
- .mat files with Unity's built-in skybox shaders import.
Materials take per-shader property defaults (Panoramic's exposure is 1, Procedural's 1.3).
3D physics builds colliders in creation order, not hash order. Contact callback order no longer depends on handle values, as 2D physics already did.
render_settings now also records the skybox shaders' defaults, a script-set skybox, a scene's own skybox material (NESky.mat), and clearing it.
GPU tests check the procedural sky against its CPU model (within 3/255), the panorama's orientation, and the six faces with rotation. MCP e2e tests cover editing, playing and saving RenderSettings, and cameras showing the default, a tinted and no skybox.
Not yet: Skybox/Cubemap (no cubemap textures yet), the Skybox component on cameras, reflection probes, and fog on the skybox (Unity doesn't fog it either).
Next: cubemap textures (Cubemap, .hdr/.exr import), then reflection probes.