feat(editor): add hierarchy_panel with entity tree display and selection

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/voltex_editor/Cargo.toml

@@ -6,5 +6,6 @@ edition = "2021"
 [dependencies]
 bytemuck = { workspace = true }
 voltex_math = { workspace = true }
+voltex_ecs = { workspace = true }
 voltex_renderer = { workspace = true }
 wgpu = { workspace = true }