Feature: full-width inline content at an anchored line in split diffs

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

Let consumers render their own content inline at a chosen line in a split diff, spanning both columns rather than sitting on the deletions or additions side.

In the stacked/narrow layout, the same content would appear inline at that line in reading order and fit the available width.

It would behave like other rendered React content:

• Normal mount and unmount.
• Its height can change after it renders, including on resize.
• It stays correctly positioned while the diff is virtualized.
• Scroll anchoring stays stable.

Motivation and context

I'm building a browser code-review UI on @pierre/diffs (currently 1.4.3). When a reviewer selects a line, the UI opens a compact, read-only preview at that spot, such as related code or a note, while keeping the surrounding diff in view.

In split mode, the preview needs the full diff width. Confined to one side, it's crawded, and it reads as belonging to only the old or new version.