feat(merge): in-app 3-way merge editor (chunk 2b)

Replace the whole-file conflict resolver with a real 3-way merge editor
built on the line-level hunk pipeline.

- ConflictModels: MergeFile/MergeFileSegment/MergeConflictBlock with
  Compose() that reassembles stable text + chosen resolutions
- ConflictResolverViewModel (same seam contract): loads conflict
  documents, flattens conflicts for one-at-a-time navigation, per-block
  Accept Ours/Base/Theirs/Both + editable result, binary files block continue
- ConflictResolverView: 3-column Base|Ours|Theirs + editable result via
  AvaloniaEdit with TextMate syntax highlighting by file extension;
  editors synced in code-behind
- add Avalonia.AvaloniaEdit + AvaloniaEdit.TextMate + TextMateSharp.Grammars;
  AvaloniaEdit theme StyleInclude in App.axaml
- rewrite ConflictResolverViewModel tests (load/gating/compose/nav/binary/abort)

src/ClaudeDo.App/App.axaml

@@ -31,6 +31,7 @@
 
     <Application.Styles>
         <FluentTheme />
+        <StyleInclude Source="avares://AvaloniaEdit/Themes/Fluent/AvaloniaEdit.xaml" />
         <StyleInclude Source="avares://ClaudeDo.Ui/Design/IslandStyles.axaml" />
         <!-- Global defaults: every Window inherits Inter Tight + body size.
              Controls that need mono opt in via their own class/style. -->