feat(merge): Rider-style 3-pane conflict editor view

Replace the Base|Ours|Theirs read-only columns + single-conflict result with a
whole-file 3-pane editor: Ours (read-only) | editable Result | Theirs (read-only),
reconstructed from the active file's segments so the panes line up on stable text.

- IBackgroundRenderer paints each conflict block (unresolved=blood, resolved=green)
  across all three panes.
- Result document edits are gated by an IReadOnlySectionProvider (stable text is
  read-only; only conflict regions, tracked via TextAnchors, are editable); edits
  flow back to the owning block.
- Between-pane gutters host inline accept controls (>/< ) positioned per conflict;
  click accepts ours/theirs into the result.
- Proportional synced vertical scroll across the panes; file switcher + change-nav
  arrows (F8 / Shift+F8); active-file 'M conflicts - K resolved' readout.
- Merge block tints + AmberBrush tokens; en/de keys for the new labels.

Seam unchanged. App builds; Ui.Tests 128, Localization.Tests 16.

src/ClaudeDo.Ui/ViewModels/Conflicts/ConflictResolverViewModel.cs

@@ -57,6 +57,12 @@ public sealed partial class ConflictResolverViewModel : ObservableObject
         OnPropertyChanged(nameof(ActiveTheirsText));
         OnPropertyChanged(nameof(ActiveResultText));
         OnPropertyChanged(nameof(PositionText));
+        // Keep the focused conflict inside the active file (e.g. when switched via the file picker).
+        if (value is not null && (Current is null || !value.Conflicts.Contains(Current)))
+        {
+            var idx = _flat.FindIndex(x => x.File == value);
+            if (idx >= 0) MoveTo(idx);
+        }
     }
 
     public string ActiveOursText   => ActiveFile?.OursText   ?? "";