Multi-select + drag-drop reorder (dnd-kit)

[tayebmokni]

Summary

Implement multi-select (Shift-click ranges, Cmd/Ctrl-click toggles) and drag-drop reordering with @dnd-kit/core. Multi-selection toolbar exposes Group, Delete, Copy, Convert to Reusable. Drag handle appears on hover; drop zones above/below siblings and into container blocks' inner-blocks area.

Design reference

• docs/04-block-editor.md §3.5 (Multi-select)
• docs/04-block-editor.md §3.6 (Drag-drop reordering)
• docs/04-block-editor.md §15.1 (Keyboard equivalent for drag)

Acceptance criteria

• Shift-click selects a contiguous range of siblings
• Cmd/Ctrl-click toggles individual selection
• Multi-selection toolbar: Group, Delete, Copy, Convert to Reusable
• @dnd-kit/core integration with drag handle in the gutter on hover
• Drop zones: above sibling, below sibling, into container inner-blocks
• Single insertion marker during drag
• Drop commits a single moveBlock action (no chained re-renders)
• Keyboard equivalent: "Move up", "Move down", "Move to position…" actions
• Drag-drop in document outline (Block inspector sidebar (Block + Document tabs) #103) also wired via dnd-kit
• a11y: dnd-kit KeyboardSensor configured per docs

Dependencies

#79, #103

Complexity

L

[tayebmokni]

Closing — shipped in Wave P (#465/#466). PR body's auto-close only fired for the first issue.