Auto-remove empty rows when deselected without editing

When a user clicks "New" to add a row, an empty row is created and its title cell receives focus. If the user then deselects the row without having edited any field, the empty row lingers in the collection.

**Proposed behavior:** auto-purge the row on deselect if no field was touched.

**Open question:** This is fundamentally a UX decision. Auto-purging matches Notion's behavior, but there's an argument for keeping the row around (the user might intend to come back to it). We should align on which behavior we want before implementing.

An e2e test codifying the auto-purge expectation is up in the linked PR.

First resolve the UX question of whether an untouched row should be purged on deselect, then inspect the New-row and deselection flow described in the issue. Review the e2e test referenced as being in the linked PR; done means the chosen behavior is implemented and the test reflects it.

Written by the indexing model from the issue text.