Path nodes should be able to edit multiple vector table rows

The *Path* (vector modify) node must store modifications for each shape's element ID now that we support multiple rows of vector data thanks to the work in #1834.

For now, we can do this based on index:

> you'd need a `HashMap` which would require refactoring all of the tools

(we don't have `RowId` yet, so we can use `usize` for its row index instead: `HashMap`)

In the future, we can attempt a smarter way of generating unique IDs: #1821.

Related: #1929.

Start by locating the Path (vector modify) node and how it stores VectorModification data, then review the multiple-row vector work in #1834. Update the storage so each shape element index retains its own modification; done means edits to multiple vector table rows remain independent.

Written by the indexing model from the issue text.