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/Views/Conflicts/ConflictResolverView.axaml

@@ -7,7 +7,7 @@
         x:DataType="vm:ConflictResolverViewModel"
         x:Class="ClaudeDo.Ui.Views.Conflicts.ConflictResolverView"
         Title="{loc:Tr conflictResolver.windowTitle}"
-        Width="1120" Height="760" MinWidth="840" MinHeight="540"
+        Width="1280" Height="820" MinWidth="960" MinHeight="560"
         CanResize="True"
         WindowDecorations="BorderOnly"
         ExtendClientAreaToDecorationsHint="True"
@@ -17,6 +17,8 @@
 
   <Window.KeyBindings>
     <KeyBinding Gesture="Escape" Command="{Binding AbortCommand}"/>
+    <KeyBinding Gesture="F8" Command="{Binding NextCommand}"/>
+    <KeyBinding Gesture="Shift+F8" Command="{Binding PreviousCommand}"/>
   </Window.KeyBindings>
 
   <Window.Styles>
@@ -24,13 +26,38 @@
       <Setter Property="FontFamily" Value="{StaticResource MonoFont}" />
       <Setter Property="FontSize"   Value="{StaticResource FontSizeMono}" />
       <Setter Property="Background" Value="{DynamicResource Surface2Brush}" />
-      <Setter Property="Padding"    Value="6,4" />
-      <Setter Property="WordWrap"   Value="True" />
+      <Setter Property="Padding"    Value="4,2" />
+      <Setter Property="WordWrap"   Value="False" />
     </Style>
     <Style Selector="Border.col-head">
       <Setter Property="Padding" Value="8,4" />
       <Setter Property="BorderThickness" Value="0,0,0,1" />
       <Setter Property="BorderBrush" Value="{DynamicResource LineBrush}" />
+      <Setter Property="Background" Value="{DynamicResource Surface2Brush}" />
+    </Style>
+    <Style Selector="Border.pane">
+      <Setter Property="BorderBrush" Value="{DynamicResource LineBrush}" />
+      <Setter Property="BorderThickness" Value="1" />
+      <Setter Property="CornerRadius" Value="6" />
+      <Setter Property="ClipToBounds" Value="True" />
+    </Style>
+    <!-- Inline accept controls in the between-pane gutters -->
+    <Style Selector="Button.accept-gutter">
+      <Setter Property="Width" Value="22" />
+      <Setter Property="Height" Value="20" />
+      <Setter Property="Padding" Value="0" />
+      <Setter Property="FontSize" Value="13" />
+      <Setter Property="FontWeight" Value="Bold" />
+      <Setter Property="HorizontalContentAlignment" Value="Center" />
+      <Setter Property="VerticalContentAlignment" Value="Center" />
+      <Setter Property="Background" Value="{DynamicResource Surface2Brush}" />
+      <Setter Property="BorderBrush" Value="{DynamicResource MergeConflictEdgeBrush}" />
+      <Setter Property="BorderThickness" Value="1" />
+      <Setter Property="CornerRadius" Value="4" />
+      <Setter Property="Foreground" Value="{DynamicResource TextBrush}" />
+    </Style>
+    <Style Selector="Button.accept-gutter:pointerover /template/ ContentPresenter">
+      <Setter Property="Background" Value="{DynamicResource MergeConflictTintBrush}" />
     </Style>
   </Window.Styles>
 
@@ -49,7 +76,7 @@
       </Grid>
     </ctl:ModalShell.Footer>
 
-    <Grid Margin="14,10" RowDefinitions="Auto,Auto,Auto,*,Auto,Auto,*">
+    <Grid Margin="14,10" RowDefinitions="Auto,Auto,Auto,*">
 
       <!-- Busy / error -->
       <TextBlock Grid.Row="0" Classes="meta" Margin="0,0,0,6"
@@ -65,7 +92,7 @@
               IsVisible="{Binding HasBinaryFiles}">
         <StackPanel Spacing="3">
           <TextBlock Classes="meta" Foreground="{DynamicResource BloodBrush}"
-                     Text="Binary files can't be merged here — abort and resolve them in your editor:"/>
+                     Text="{loc:Tr conflictResolver.binaryHint}"/>
           <ItemsControl ItemsSource="{Binding BinaryFilePaths}">
             <ItemsControl.ItemTemplate>
               <DataTemplate x:DataType="x:String">
@@ -76,69 +103,64 @@
         </StackPanel>
       </Border>
 
-      <!-- Navigation header -->
-      <Grid Grid.Row="2" ColumnDefinitions="Auto,Auto,*,Auto" Margin="0,0,0,8"
+      <!-- Toolbar: change nav · file switcher · readout -->
+      <Grid Grid.Row="2" ColumnDefinitions="Auto,Auto,Auto,*,Auto" Margin="0,0,0,8"
             IsVisible="{Binding HasCurrent}">
-        <Button Grid.Column="0" Classes="btn" Content="◀ Prev" Margin="0,0,6,0"
+        <Button Grid.Column="0" Classes="btn" Content="↑" Margin="0,0,4,0" Padding="10,4"
+                ToolTip.Tip="{loc:Tr conflictResolver.prevConflict}"
                 Command="{Binding PreviousCommand}"/>
-        <Button Grid.Column="1" Classes="btn" Content="Next ▶"
+        <Button Grid.Column="1" Classes="btn" Content="↓" Margin="0,0,12,0" Padding="10,4"
+                ToolTip.Tip="{loc:Tr conflictResolver.nextConflict}"
                 Command="{Binding NextCommand}"/>
-        <TextBlock Grid.Column="2" Classes="path-mono" VerticalAlignment="Center"
-                   Margin="14,0" TextTrimming="CharacterEllipsis"
-                   Text="{Binding CurrentPath}"/>
-        <TextBlock Grid.Column="3" Classes="meta" VerticalAlignment="Center"
+        <ComboBox Grid.Column="2" MinWidth="240" MaxWidth="520"
+                  ItemsSource="{Binding Files}"
+                  SelectedItem="{Binding ActiveFile, Mode=TwoWay}">
+          <ComboBox.ItemTemplate>
+            <DataTemplate x:DataType="vm:MergeFile">
+              <TextBlock Classes="path-mono" Text="{Binding Path}" TextTrimming="CharacterEllipsis"/>
+            </DataTemplate>
+          </ComboBox.ItemTemplate>
+        </ComboBox>
+        <TextBlock Grid.Column="4" Classes="meta" VerticalAlignment="Center"
                    Foreground="{DynamicResource TextDimBrush}"
                    Text="{Binding PositionText}"/>
       </Grid>
 
-      <!-- Three-way columns: Base | Ours | Theirs -->
-      <Grid Grid.Row="3" ColumnDefinitions="*,*,*" IsVisible="{Binding HasCurrent}">
-        <Border Grid.Column="0" BorderBrush="{DynamicResource LineBrush}" BorderThickness="1" CornerRadius="6" Margin="0,0,4,0">
+      <!-- Three panes: Ours | (gutter) | Result | (gutter) | Theirs -->
+      <Grid Grid.Row="3" ColumnDefinitions="*,26,*,26,*" IsVisible="{Binding HasCurrent}">
+        <Border Grid.Column="0" Classes="pane">
           <DockPanel>
             <Border Classes="col-head" DockPanel.Dock="Top">
-              <TextBlock Classes="eyebrow" Text="BASE"/>
+              <TextBlock Classes="eyebrow" Text="{loc:Tr conflictResolver.ours}"
+                         Foreground="{DynamicResource MossBrush}"/>
             </Border>
-            <ae:TextEditor Name="BaseEditor" IsReadOnly="True"/>
+            <ae:TextEditor Name="OursEditor" IsReadOnly="True" ShowLineNumbers="True"/>
           </DockPanel>
         </Border>
-        <Border Grid.Column="1" BorderBrush="{DynamicResource LineBrush}" BorderThickness="1" CornerRadius="6" Margin="4,0">
+
+        <Canvas Grid.Column="1" Name="LeftGutter" Background="Transparent"/>
+
+        <Border Grid.Column="2" Classes="pane">
           <DockPanel>
             <Border Classes="col-head" DockPanel.Dock="Top">
-              <TextBlock Classes="eyebrow" Text="OURS · current (merge target)" Foreground="{DynamicResource MossBrush}"/>
+              <TextBlock Classes="eyebrow" Text="{loc:Tr conflictResolver.result}"/>
             </Border>
-            <ae:TextEditor Name="OursEditor" IsReadOnly="True"/>
+            <ae:TextEditor Name="ResultEditor" ShowLineNumbers="True"/>
           </DockPanel>
         </Border>
-        <Border Grid.Column="2" BorderBrush="{DynamicResource LineBrush}" BorderThickness="1" CornerRadius="6" Margin="4,0,0,0">
+
+        <Canvas Grid.Column="3" Name="RightGutter" Background="Transparent"/>
+
+        <Border Grid.Column="4" Classes="pane">
           <DockPanel>
             <Border Classes="col-head" DockPanel.Dock="Top">
-              <TextBlock Classes="eyebrow" Text="THEIRS · incoming (task)" Foreground="{DynamicResource AccentBrush}"/>
+              <TextBlock Classes="eyebrow" Text="{loc:Tr conflictResolver.theirs}"
+                         Foreground="{DynamicResource AmberBrush}"/>
             </Border>
-            <ae:TextEditor Name="TheirsEditor" IsReadOnly="True"/>
+            <ae:TextEditor Name="TheirsEditor" IsReadOnly="True" ShowLineNumbers="True"/>
           </DockPanel>
         </Border>
       </Grid>
-
-      <!-- Accept actions -->
-      <WrapPanel Grid.Row="4" Orientation="Horizontal" Margin="0,8" IsVisible="{Binding HasCurrent}">
-        <Button Classes="btn" Content="Accept Ours" Margin="0,0,8,0"
-                Command="{Binding Current.AcceptOursCommand}"/>
-        <Button Classes="btn" Content="Accept Base" Margin="0,0,8,0"
-                IsEnabled="{Binding Current.HasBase}"
-                Command="{Binding Current.AcceptBaseCommand}"/>
-        <Button Classes="btn" Content="Accept Theirs" Margin="0,0,8,0"
-                Command="{Binding Current.AcceptTheirsCommand}"/>
-        <Button Classes="btn" Content="Accept Both" Margin="0,0,8,0"
-                Command="{Binding Current.AcceptBothCommand}"/>
-      </WrapPanel>
-
-      <!-- Merged result -->
-      <TextBlock Grid.Row="5" Classes="eyebrow" Text="MERGED RESULT" Margin="0,4,0,4"
-                 IsVisible="{Binding HasCurrent}"/>
-      <Border Grid.Row="6" BorderBrush="{DynamicResource LineBrush}" BorderThickness="1" CornerRadius="6"
-              IsVisible="{Binding HasCurrent}">
-        <ae:TextEditor Name="ResultEditor" ShowLineNumbers="True"/>
-      </Border>
     </Grid>
   </ctl:ModalShell>
 </Window>