Fix "Go to scene hierarchy" doing nothing on non-scene assets

[Diphome]

Bug

Right-clicking a row in the Asset List and choosing Go to scene hierarchy did nothing.

Cause

The context item was made visible for every single selection, but only assets that belong to a GameObject carry a TreeNode — Components, and the Mesh referenced by a MeshFilter/SkinnedMeshRenderer. For any other asset (textures, audio, text, standalone meshes, …) TreeNode is null, so the handler's if (TreeNode != null) guard fell through and nothing happened. The item looked clickable but was a dead entry.

Fix

• Only show Go to scene hierarchy when the selected asset actually has a scene node (TreeNode != null), so it's never a dead item.
• Make the jump robust: switch to the Scene Hierarchy tab first, select the node, EnsureVisible() (expand ancestors + scroll into view), then focus the tree.
• Read the selection from the virtual list's backing list (visibleAssets[i]) instead of assetListView.Items[i].

Verified

• net9.0-windows build green. CI validates net472/net8/net9.
• ⚠️ Not yet clicked through on a live project with a scene — if you load one, right-click a Transform/Component or a scene Mesh: the item should appear and jump+scroll to its node; on a texture/audio asset the item should now be hidden rather than dead.