Add semantic highlighting to the Local History compare editor

The PR #2930 adds semantic highlighting to the Java compare/merge viewer, which works for the EGit Revision side (using `JavaMergeViewer` / `CompilationUnitEditorAdapter`).

However, the **Local History** compare editor still lacks semantic highlighting. When comparing a file with its local history, the editor only shows basic syntax coloring (keywords, strings, comments) without the richer highlighting (method names, type references, local variables, fields, type parameters, annotations, etc.).

This is a follow-up to #2929 to extend semantic highlighting support to the Local History compare editor as well.

Related:
- #2929 - Original feature request
- #2930 - PR addressing the EGit Revision side

Start by reading PR #2930 and the Java compare/merge viewer path involving JavaMergeViewer and CompilationUnitEditorAdapter. Trace the corresponding Local History compare editor entry point, then verify that semantic highlighting covers method names, type references, local variables, fields, type parameters, and annotations rather than only basic syntax coloring.

Written by the indexing model from the issue text.