SceneOps: Slice F — Move entity to different scene node + improved node-reparent UX

[fernandotonon]

Parent epic: #623

Goal

The other half of the user's hierarchy request: move a whole mesh (entity) into a different scene node, and tighten the existing scene-node reparent (already present via SceneTreeModel::reparentNode) so users can actually find and use it.

Scope

Entity move

• New helper SubMeshOps::moveEntityToNode(entity, targetNode, transformMode) -> bool where transformMode ∈ { keepWorld, keepLocal }.
• Same convention as the bone-reparent in Slice B of Epic: Skel — Skeletal rigging & control #554:
  • Keep world (default): recomputes the entity's scene node's local TRS so the world transform is preserved.
  • Keep local: TRS unchanged; the entity visibly jumps to the new parent's space.
• Multi-select: dragging multiple entities to a new parent preserves each one's world transform individually.
• Undo: MoveEntityCommand stores the original parent + local TRS.

Node reparent UX polish

• SceneTreeModel::reparentNode is already implemented but invisible to most users. Surface it:
  • Drag-and-drop a scene node onto another node in qml/SceneTreeNode.qml triggers reparent (with the same keep-world / keep-local picker on first use; remembered as a session default).
  • Right-click context menu: "Set parent…" picker dialog as a fallback.
  • Drag onto empty space (or onto the "Scene Root" entry at the top) detaches the node to root.
• Validation feedback: the existing canReparent is used to highlight valid drop targets in green / invalid in red during drag, with a tooltip explaining "circular parent rejected" / "already a child" cases.
• Group affordance: if multiple nodes are selected and the user picks "Group selected" from the context menu, route through the existing Manager::groupNodes so the new affordance shares the proven group-node path.
• Ungroup affordance: right-click on a group node → "Ungroup" via existing Manager::ungroupNode.
• Visual indicator for group nodes vs regular nodes in the tree (small icon distinguishing them).

Combined

• Drag protocol: the scene tree learns a tiny drop-target taxonomy:
  • SubEntity dragged onto Entity → move submesh (Slice B).
  • Entity dragged onto SceneNode → move entity (this slice).
  • SceneNode dragged onto SceneNode → reparent (this slice).
  • Multiple SceneNodes dragged onto empty space → group (this slice via Manager::groupNodes).
  • The drop handler dispatches by source/target type.

Acceptance Criteria

• Dragging an entity onto a different scene node moves it, with keep-world preserving the visual position.
• Keep-local toggles the entity's apparent position correctly.
• Multi-entity reparent preserves each entity's world transform.
• Dragging a scene node onto another reparents via SceneTreeModel::reparentNode, with canReparent validation surfaced as drop-target highlight + tooltip.
• Drop on root (empty space or "Scene Root" entry) detaches.
• Group / Ungroup affordances surface in the right-click menu and route through Manager::groupNodes/ungroupNode.
• Drop taxonomy correctly dispatches SubEntity / Entity / SceneNode drops.
• Undo works for entity-move; existing undo for node reparent is verified.
• Project save / load round-trips the new hierarchy.
• Sentry breadcrumbs scene.tree.entity_move, scene.tree.node_reparent, scene.tree.group, scene.tree.ungroup.
• Headless-CI test: entity move (keep-world + keep-local), node reparent, group of 3 nodes.

Effort

~5 days.