[codex] Preserve scroll rows during drag

[nexiumbiz-debug]

Summary

• keep scrollVertical row count from shrinking while an item is being dragged or resized
• still allow rows to grow during the active interaction
• allow the grid to shrink back to the actual content height once the interaction ends
• add component coverage for the mid-drag preserve and post-drag shrink behavior

Fixes #794.

Root cause

setGridDimensions() recalculates row count from the current item positions on every layout update. In scrollVertical, dragging an item upward can reduce the computed row count while the pointer is still inside the grid, which shrinks the grid underneath the drag and amplifies the movement.

Validation

• npx prettier --check projects/angular-gridster2/src/lib/gridster.ts projects/angular-gridster2/src/lib/tests/gridster.spec.ts
• npx eslint projects/angular-gridster2/src/lib/tests/gridster.spec.ts (only the pre-existing Node module-type warning)
• npm run test-lib -- --watch=false
• npm run build-lib
• git diff --check (only the repo's LF/CRLF warning)