## Summary
This PR adds the current Stage Godot avatar presentation path:
avatar-only same-frame glow plus tuned toon color mapping.
<img width="994" height="951" alt="屏幕截图 2026-06-02 015159"
src="https://github.com/user-attachments/assets/feb94b95-745f-4b47-aba2-0289dafecd1c"
/>
<img width="1769" height="1279" alt="屏幕截图 2026-06-02 021212"
src="https://github.com/user-attachments/assets/d4437dc2-e859-42ca-b3fd-0a90e55d641b"
/>
## Design
The implementation uses three layers:
1. Base avatar material rendering still comes from the vendored V-Sekai
MToon shader.
2. `StageAvatarGlowRuntime` marks visible avatar meshes through a
depth-tested `MaterialOverlay` stencil pass.
3. `StageAvatarGlowCompositorEffect` reads that stencil mask in the same
frame, extracts avatar pixels, builds the glow pyramid, composites
glare, and applies the current NAES/toon color mapping.
This avoids Godot Environment Glow because that path is global. Here,
source selection needs to be avatar-specific and controlled by the stage
runtime.
It also avoids an extra SubViewport path because prior testing showed
camera-motion lag. The compositor path keeps the effect in the same
render frame.
## Stage Baseline
`StageVisualPreset` now keeps the stage environment neutral for this
effect:
- skybox is visible as background only
- skybox does not drive avatar/ground ambient light
- reflected light is disabled
- Godot Environment Glow is disabled
- Godot tonemap/adjustment stay neutral
- custom color mapping runs in the compositor
## Notes
`StageAvatarGlowCompositorEffect` currently owns both avatar glow and
toon color mapping. That is acceptable for this PR because it is the
only custom stage compositor today.
Before adding rim light, screen-space outlines, or more post effects,
compositor ownership should be split from individual features so effects
can register passes instead of replacing `Camera3D.Compositor`
independently.
Lilia_Chen
5b0568bbb4