[codex] Fix vertical push fallback state

[nexiumbiz-debug]

Summary

• prevents recursive push fallback from treating the currently dragged item as an empty target cell
• removes abandoned pushed-item paths from the correct item path during rollback
• adds a regression test for vertical-only push where dragging the fourth stacked item upward should not swap unrelated first/second items

Fixes #941.

Validation

• npm run test-lib -- --watch=false
• npm run build-lib
• npx eslint projects/angular-gridster2/src/lib/tests/gridsterPush.spec.ts

Note: full npm run lint -- angular-gridster2 still reports existing unrelated lint errors across the project.