feat: Multi-Select for ObjectsList UI Panel

[ViktorVovk]

Multi-selection for objects and folders in the Objects panel

What

Adds multi-selection support to the Objects panel, letting users select, copy, cut, paste, duplicate, delete, and reorganize multiple objects and folders at once — including cross-scene and cross-global-objects actions.

Changes

Selection

• Ctrl+click adds/removes individual items to the selection.
• Shift+click extends the selection from the anchor using VS Code–style "multi-range" behavior: items outside the new range that were selected before the Shift press are preserved, not dropped.
• Keyboard navigation (arrow keys) without Shift moves the cursor and updates the anchor; with Shift it extends the range from the existing anchor.
• Selection is scoped to one section at a time (scene objects or global objects). Crossing the boundary is silently capped so the invariant is never violated.

Context menu on multi-selection
Shows a bulk action menu instead of the single-item menu:

• Copy / Cut / Paste / Duplicate
• Delete (with a single confirmation dialog listing the count)
• Move to folder → submenu with all valid target folders (excludes the current parent if all selected items are already there, and excludes selected folders and their descendants)
• Create new folder (moves the selection into a freshly created folder)
• Set as global object (only shown when the entire selection consists of non-folder scene objects)

Keyboard shortcuts
Ctrl+C, Ctrl+X, Ctrl+V, Ctrl+D, Backspace/Delete, Ctrl+A, Escape all operate on the full selection. Ctrl+D duplicates each selected item right after itself in its parent folder, preserving per-parent insertion offsets when siblings share a folder. Cut serializes to an in-memory payload before prompting for deletion so that a cancelled cut never pollutes the clipboard.

Clipboard — copy/paste of folders
Folders are serialized recursively (structure + all child objects with their properties and behaviors). Pasting a folder restores the full subtree, generating unique names for any name collisions. Pasting a mix of objects and folders works correctly; pasting only empty folders does not fire the onObjectCreated hook.

Drag and drop
Multiple selected items can be dragged as a unit. The drop is blocked when the destination folder is itself part of the selection (C++ only guards descendants, not self). Dropping onto the global-objects section is blocked when the selection contains folders.

Properties panel
Shows "N objects selected" when more than one plain object is selected; shows "Folder selected. No properties to display." when only folders are selected.

Bug fixes included

• onDelete shortcut no longer calls delete() on undefined when nothing is selected.
• onPaste with no selection now respects the last-active section (scene vs global).
• bulkDelete on empty-folder-only selections correctly triggers onObjectModified so unsaved-changes tracking fires.
• Pasting multiple objects no longer auto-places stacked instances at the same canvas coordinates when newObjectInstanceSceneCoordinates is stale from a previous "add under cursor" flow.
• "Move to folder" submenu no longer shows the current parent folder as a valid destination.
• bulkMoveToFolder preserves the original selection order instead of reversing it.

Architecture

New file	Responsibility
ObjectFolderOrObjectsClipboard.js	Serialize / deserialize object+folder trees to/from the OS clipboard
ObjectFolderOrObjectsDuplicate.js	In-place duplication without touching the clipboard
useBulkObjectOperations.js	Copy, cut, paste, duplicate, delete, set-as-global, move-to-folder, build bulk menu
useObjectsSelection.js	Selection state, section tracking, select-all, deselect-all
useTreeViewSelection.js	Anchor-based Shift+click / Shift+arrow multi-range selection for TreeView

Selection utilities (getSelectionTopLevelNodes, getObjectsToDeleteFromSelection, removeEmptyFoldersFromSelection, enumerateAllChildrenInFolder) live in EnumerateObjectFolderOrObject.js, separate from clipboard concerns.

TreeView/index.js gained three exported types — FlattenedNode<Item>, SelectArgs<Item>, OnSelectFn<Item> — that eliminate the duplicated inline shapes across ItemData, getItemProps, and useTreeViewSelection.

Testing

• ObjectFolderOrObjectsClipboard.spec.js — unit tests for serialization round-trips, nested-folder paste, duplicate-name resolution, backward compatibility with the single-item clipboard format, and in-place duplication.
• ObjectFolderOrObjectSelection.spec.js — extended with multi-selection scenarios.
• ObjectsList.stories.js — new WithMultiSelection story.