Phase 1 of #181: read-only side-by-side Compare. A/B toggle renders the existing sidebar twice — live working copy (A) + a loaded snapshot (B), B labeled with its own name + diff-tinted (green=only in comparison, red=same name/graph differs ignoring savedAt). Non-destructive: read-only capture guard on B, onChoose chooser never calls applySnapshot. Phase 2 (pull-in + A<->B swap) tracked in #197.