Adds an additive multi-selection layer alongside the existing single-select
state for both standalone elements and Address Controls: rubber-band drag,
Ctrl/Shift-click toggle, group drag preserving relative offsets, per-item
highlight, and a disabled "N items selected" properties panel. Collapses
back to ordinary single-select whenever 0-1 items remain selected so every
existing code path is unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>