Support Shift+wheel scrolling in combined diffs

Combined diff sections currently lack the Shift+wheel listener needed for horizontal scrolling. Reimplement the focused editor behavior with adapter regression tests. Source PR https://github.com/stablyai/orca/pull/11756; credit @brohoya. No contributor action requested.

Start by reviewing the combined diff sections and the focused editor behavior referenced in the issue, then inspect the adapter regression-test area. Compare the implementation with source PR #11756; done means combined diffs support Shift+wheel horizontal scrolling and adapter regression coverage exists.

Written by the indexing model from the issue text.