Unified Diff: Add overview ruler markers for diff hunks #2941

Problem

The classic side-by-side TextMergeViewer provides a bird's-eye view next to its vertical scrollbar. It shows the location and distribution of all differences across the complete document and allows users to navigate directly to a difference.

The unified diff presentation embedded in a text editor currently highlights differences in the visible editor area but does not show corresponding markers in the editor overview ruler. Consequently, off-screen differences are difficult to discover and navigate.

Expected Behavior

Add overview ruler markers for top-level unified-diff hunks. Reuse the existing unified-diff annotations and the host editor's overview ruler where possible. Detailed/token-level diff annotations should not produce additional markers.