fix(diff): label split panes, unify the mode switch, sync split scrolling

Three things were wrong with the fresh side-by-side viewer:

- The panes scrolled independently. The sync hunted the editors' templated
  ScrollViewers at load time, but the viewer starts collapsed (no file selected),
  so nothing was ever measured, no template existed, and the hook silently
  no-opped for the lifetime of the window. Reading now goes through
  TextView.ScrollOffsetChanged, which exists from construction; writing goes
  through a lazily resolved ScrollViewer.Offset. TextEditor.ScrollToVerticalOffset
  is NOT usable here - it is a silent no-op in AvaloniaEdit 12.0.0 even with the
  editor templated (verified headlessly).
- Neither pane said what it showed. Added a BASE | WORKTREE header row inside
  DiffTextView, sharing the pane grid's columns so each label sits over its editor.
- The layout picker was two lookalike ToggleButtons. It is now a segmented switch
  (Border.segmented + Button.segment.active), with wrap demoted to an icon toggle
  since it is orthogonal to the layout mode.

The scroll sync, TextMate setup, grammar switching, brush fallback and segment
struct were duplicated between this control and the 3-pane conflict resolver,
which the design doc had used as a copy-paste template. They now live in
DiffEditorSetup; both surfaces migrated. They stay separate controls on purpose -
a read-only two-way renderer over aligned rows with filler lines is not a variant
of a three-way editor over a writable document.

The resolver thereby also picks up the fixed scroll sync and the TryFindResource
brush lookup (its TryGetResource never resolved anything). All four merge tokens
happen to equal their hardcoded fallbacks, so nothing changes on screen.

src/ClaudeDo.Localization/locales/en.json

@@ -358,8 +358,11 @@
       "filesHeader": "Files",
       "binary": "Binary file — no text diff",
       "empty": "No content",
+      "unifiedView": "Unified",
       "splitView": "Side by side",
-      "wrapLines": "Wrap lines"
+      "wrapLines": "Wrap lines",
+      "paneBase": "BASE",
+      "paneWorktree": "WORKTREE"
     },
     "worktreesOverview": {
       "refresh": "Refresh",