feat: add nemo-bevy crate for Bevy 3D embedding in GPUI

[geoffjay]

Summary

• Adds nemo-bevy crate implementing a headless Bevy 0.15 renderer on a background thread that streams RGBA frames to GPUI via ImageSource::Custom + RenderImage
• Adds BevyScene GPUI component with continuous repaint (request_animation_frame), mouse move/click, and scroll wheel input forwarding
• Feature-gated behind bevy feature flag on the nemo crate — existing builds are unaffected

Architecture

GPUI main thread                        Bevy background thread
──────────────                          ─────────────────────
BevyScene component                     App::new() headless
  └── img(ImageSource::Custom(..))        + ScheduleRunnerPlugin (60fps)
        └── reads LatestFrame               + ImageCopyPlugin (render graph)
              ↑                               + Camera → RenderTarget::Image
              └──── crossbeam channel ←──── texture → buffer → Vec<u8>

New files

• crates/nemo-bevy/ — standalone crate with BevyRenderer, FrameData, LatestFrame, BevyInputEvent
• crates/nemo/src/components/bevy_scene.rs — BevyScene + BevySceneState GPUI component

Modified files

• Workspace Cargo.toml — added member + dependency
• crates/nemo/Cargo.toml — optional nemo-bevy dep + bevy feature
• crates/nemo/src/components/{mod,state}.rs — cfg-gated module + ComponentState::Bevy variant
• crates/nemo/src/app.rs — get_or_create_bevy_state() + "bevy_scene" render arm
• crates/nemo-registry/src/builtins.rs — registered bevy_scene with width/height schema

Test plan

• cargo check -p nemo-bevy — standalone compilation
• cargo check -p nemo --features bevy — integration compilation
• cargo check -p nemo — existing build unaffected (no bevy feature)
• cargo test -p nemo-bevy — 4 unit tests pass (frame conversion, LatestFrame store/take/peek)
• cargo test -p nemo-registry — builtins registration test passes with new component
• Manual: create XML config with <bevy_scene> and verify spinning cube renders in GPUI window

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