fix: guard null selected item during item tool placement

[open-pascal]

Summary

• allow the item placement coordinator to receive a nullable asset
• bail out early in the placement effects/frame loop when no asset is selected
• remove the non-null assertions in item-tool.tsx

Why

This ports the recent monorepo fix from Anton (174246fb, fix(editor): guard null asset in usePlacementCoordinator).

The OSS editor still had the older partial guard, but it was still assuming selectedItem! in item-tool.tsx and still running placement coordinator effects/frames when the asset was null. That can happen during transient tool activation or scene updates and can cascade into render failures.

Verification

• npx -y @biomejs/biome check packages/editor/src/components/tools/item/item-tool.tsx packages/editor/src/components/tools/item/use-placement-coordinator.tsx
• npx -y react-doctor@latest . --verbose --diff (packages/editor scored 99/100, warning-only on pre-existing unknown properties in use-placement-coordinator.tsx)