feat: enable native readonly diff interaction and synchronized pane cursors

src/main.rs

@@ -44,6 +44,8 @@ Buffers: :edit PATH, :buffers, :buffer N, :buffer_next, :buffer_previous, :buffe
 Markdown links: :follow_link (primary caret; local files only)
 Diff: :diff (Normal); choose Before then After; Working checkout After edits live unsaved buffers
       Before and historical/remote After stay read-only; Tab/Shift+Tab categories/Normal focus
+      All text panes use native motions, selections and copy; read-only panes reject edits/saves
+      Switching Before/After preserves the aligned cursor row and display column
       Type to search IDs/refs; navigation skips omitted lines without expanding excerpts
       Normal [/] explicitly expands context; Insert brackets remain literal text
       :diff.close exits explicitly; Esc cancels modes/prompts/filtering without closing the diff