feat: Add demo scenes for testing 3D raycast collider annotations

[hatayama]

Summary

• Adds two self-contained demo scenes for exercising screenshot --annotate-raycast-grid and the raycast tool: a flat 4x4 tile collider grid, and a perspective-camera scene with split/rotated-L collider shapes.
• Both scenes include a UI blocker overlay so the UI-occlusion handling added in C2/C3 (covered PhysicsCollider entries get omitted from AnnotatedElements) has something concrete to verify against.

User Impact

• Before: there was no ready-made scene to try the raycast grid annotation feature (dense sampling, layer summaries, L-shaped/split collider bounds, UI occlusion) without building a test scene by hand.
• After: opening either demo scene and running screenshot --capture-mode rendering --annotate-raycast-grid true gives immediate, reproducible output to inspect.

Changes

• Assets/Scenes/RaycastAnnotationDemoScene.unity (+ .meta): flat tile grid (Tile_R0_C0..R3_C3), Main Camera, Directional Light, EventSystem, and a UI blocker Canvas.
• Assets/Scenes/RaycastAnnotationPerspectiveDemoScene.unity (+ .meta): perspective split-grid and rotated-L collider layouts with the same UI blocker setup.
• Ported unchanged from main's a2abaf04 (fix: Raycast annotations show collider coverage without covered UI targets #1500); both scenes reference only built-in Unity/URP assets with no custom scripts or external prefabs, so no other file changes were needed.
• Not registered in ProjectSettings/EditorBuildSettings.asset — main doesn't register them either, matching the existing pattern for this repo's other manually-opened demo scenes (SimulateKeyboardDemoScene, SimulateMouseDemoScene).

Verification

• uloop compile → 0 errors, 0 warnings
• Confirmed both scenes' .meta GUIDs don't collide with any existing v3-beta asset
• Confirmed every guid: reference inside both scene files resolves to a built-in Unity/URP asset already present in this project (no custom scripts, prefabs, or materials referenced)

[coderabbitai]

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• Assets/Scenes/RaycastAnnotationPerspectiveDemoScene.unity is excluded by !**/*.unity and included by none
• Assets/Scenes/RaycastAnnotationPerspectiveDemoScene.unity.meta is excluded by none and included by none

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/raycast-demo-scenes

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