Excalidraw sync drops legitimate updates due to fragile checksum heuristic

[zxch3n]

examples/excalidraw-example/src/hooks/useLoroSync.ts:210-218

• The hook sums element version fields and treats equal sums as “no change,” returning early. Any scene
  change that keeps the sum constant (e.g. swapping shapes with same version, many app-state changes, or
  attribute tweaks that don’t bump version) gets skipped and never written into the CRDT.
• Collaborators then miss color/theme toggles or reordered elements, even though CRDT state actually
  changed.
• Fix: Remove the checksum guard or replace it with a real diff keyed by element ID + version/app-state
  hash. Let Loro’s own dedup handle identical imports instead of this false-negative-prone shortcut.