feat(editor): multi-actor selection in Outliner

[JeanPhilippeKernel]

Summary

The Outliner currently supports single-actor selection only (SelectedActorHandle on the scene). Add Ctrl+click / Shift+click multi-selection with a unified gizmo.

Requirements

Selection model

• Keep SelectedActorHandle as the primary selection (gizmo anchor, property panel target).
• Add an editor-side Array<ActorHandle> m_selected_handles in HierarchyViewUIComponent (no scene change needed for phase 1).
• Ctrl+click — toggle an actor into/out of the selection set; make it the primary if it's the only one selected.
• Shift+click — range-select from the last clicked actor to the current one (DFS order).
• Click with no modifier — clear selection, select clicked actor only.

Visual feedback

• Highlight all selected rows (same COL_SELECTED tint used today).
• Mark the primary actor with a slightly brighter tint or outline.

Gizmo

• When more than one actor is selected, compute a centroid world position from all selected WorldTransform translation columns.
• Show ImGuizmo at the centroid; on drag, distribute the delta (translation/rotation/scale) to each actor's local Position/Rotation/Scale.
• For rotation and scale deltas, apply them in each actor's local frame (multiply local matrix by delta matrix, decompose).

Keyboard

• Escape — deselect all.
• Delete — delete all selected actors.

Out of scope (follow-up)

• Multi-selection drag-and-drop reparent.
• Copy/paste of selection.

Files likely touched

• Tetragrama/Components/HierarchyViewUIComponent.h — add m_selected_handles, m_last_clicked_idx
• Tetragrama/Components/HierarchyViewUIComponent.cpp — click handling, row highlight, gizmo centroid logic