feat(stage-tamagotchi-godot): G1.3 default visual baseline (#1904)

## Summary

Add the G1.3 default visual baseline for `stage-tamagotchi-godot`.

This moves the Godot stage from a model-loading/runtime skeleton toward
a default presentation stage: fixed sky environment, visual grid ground,
centre marker, fixed lighting rig, camera ground constraint, and focused
material/rendering verification.

## Changes

- Add a fixed runtime visual preset for the Godot stage
  - WorldEnvironment with the existing stage-ui-three HDRI skybox
  - visual-only grid ground at world `Y=0`
  - centre `T` marker at the world origin
  - fixed directional light rig replacing the old single `OmniLight3D`

- Reuse the existing three-stage HDRI asset
- resolve
`packages/stage-ui-three/src/components/Environment/assets/sky_linekotsi_23_HDRI.hdr`
from a workspace checkout
  - do not copy or move the asset into Godot `assets/`
  - keep release packaging for the shared HDRI as a follow-up

- Add camera ground constraint in view-state rules
  - clamp camera position `Y` during view-state normalisation / commit
- applies uniformly to local input, settings-driven patches, and future
remote/agent patches

- Patch vendored Godot MToon shader behaviour
  - disable implicit WorldEnvironment ambient light for MToon materials
- use Godot alpha scissor / alpha-to-coverage path for cutout MToon
variants
- document the local vendor patch and removal conditions in
`docs/vendor-patches.md`

- Improve Godot rendering defaults
  - enable 3D MSAA
  - render 3D at 2x scale to avoid low-DPI/viewport scaling aliasing

- Add verification harnesses
- C# engine-local checks for camera ground constraint and HDRI asset
resolution
- Godot material rendering check scene for AvatarSample A/B material
import coverage

- Clean stage root presentation
  - remove the old on-screen runtime status label
  - keep Electron bridge and scene/view runtime wiring intact

## Notes

The HDRI path resolution is intentionally workspace-only in this PR.
Release packaging for sharing the three-stage HDRI with the exported
Godot sidecar remains a follow-up.

The MToon shader changes are deliberate vendor patches, recorded in
`docs/vendor-patches.md`, because `ambient_light_disabled`,
`alpha_to_coverage`, and alpha scissor behaviour are shader-level
changes rather than runtime material parameters.

The material rendering check validates imported material structure, not
visual golden output.

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>