fix(merge): unresolved conflicts compose to empty, not Ours (+ review nits)

Code-review follow-ups before push:
- MergeFile.ResultText/Compose() fell back to Ours for unresolved conflicts while
  the editor seeds them empty — align both on empty so the public model matches the
  pane and Continue can't silently auto-accept Ours.
- Bound the gutter re-layout retry (was an unbounded Background re-post when the
  editor isn't laid out, e.g. minimized).
- Pluralize the readout ('1 conflict' not '1 conflicts'). Tests updated. Ui 128 green.

tests/ClaudeDo.Ui.Tests/ViewModels/ConflictResolverViewModelTests.cs

@@ -238,7 +238,7 @@ public class ConflictResolverViewModelTests
 
         Assert.Equal("a\no\nz\n", file.OursText);
         Assert.Equal("a\nt\nz\n", file.TheirsText);
-        Assert.Equal("a\no\nz\n", file.ResultText);   // unresolved seeds Ours
+        Assert.Equal("a\nz\n", file.ResultText);   // unresolved conflicts start empty
 
         // After resolving: ResultText reflects the resolution
         file.Conflicts[0].Resolution = "r\n";
@@ -286,7 +286,7 @@ public class ConflictResolverViewModelTests
         // Switch to file B
         vm.SelectFileCommand.Execute(vm.Files[1]);
         Assert.Equal("b.cs", vm.ActiveFile!.Path);
-        Assert.Equal("ours-b\n", vm.ActiveResultText);   // unresolved seeds Ours
+        Assert.Equal("", vm.ActiveResultText);   // unresolved conflicts start empty
 
         // Switch back to file A
         vm.SelectFileCommand.Execute(vm.Files[0]);
@@ -303,11 +303,11 @@ public class ConflictResolverViewModelTests
         await vm.OpenAsync("main");
 
         // 1 conflict, 0 resolved
-        Assert.Equal("1 conflicts · 0 resolved", vm.PositionText);
+        Assert.Equal("1 conflict · 0 resolved", vm.PositionText);
 
         vm.Current!.AcceptOursCommand.Execute(null);
 
         // 1 conflict, 1 resolved
-        Assert.Equal("1 conflicts · 1 resolved", vm.PositionText);
+        Assert.Equal("1 conflict · 1 resolved", vm.PositionText);
     }
 }