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.Ui/ViewModels/Conflicts/ConflictModels.cs

@@ -5,45 +5,74 @@ using CommunityToolkit.Mvvm.Input;
 
 namespace ClaudeDo.Ui.ViewModels.Conflicts;
 
-public sealed partial class ConflictHunk : ObservableObject
+/// <summary>
+/// One conflict region in a file: the two competing versions (and the merge base when the
+/// merge used diff3 style), plus the chosen <see cref="Resolution"/> (null until resolved).
+/// </summary>
+public sealed partial class MergeConflictBlock : ObservableObject
 {
     public string Ours { get; }
-    public string Theirs { get; }
     public string? Base { get; }
+    public string Theirs { get; }
 
     [ObservableProperty] private string? _resolution;
 
     public bool IsResolved => Resolution is not null;
+    public bool HasBase => Base is not null;
 
-    public ConflictHunk(string ours, string theirs, string? @base)
+    public MergeConflictBlock(string ours, string? @base, string theirs)
     {
         Ours = ours;
-        Theirs = theirs;
         Base = @base;
+        Theirs = theirs;
     }
 
     partial void OnResolutionChanged(string? value) => OnPropertyChanged(nameof(IsResolved));
 
-    [RelayCommand] private void AcceptCurrent()  => Resolution = Ours;
-    [RelayCommand] private void AcceptIncoming() => Resolution = Theirs;
-    [RelayCommand] private void AcceptBoth()     => Resolution = Ours + Theirs;
-    [RelayCommand] private void EditManually()   => Resolution ??= Ours;
+    [RelayCommand] private void AcceptOurs()   => Resolution = Ours;
+    [RelayCommand] private void AcceptTheirs() => Resolution = Theirs;
+    [RelayCommand] private void AcceptBoth()   => Resolution = Ours + Theirs;
+    [RelayCommand] private void AcceptBase()   => Resolution = Base ?? "";
 }
 
-public sealed class ConflictFile
+/// <summary>An ordered piece of a conflicted file: either stable common text or a conflict block.</summary>
+public sealed class MergeFileSegment
 {
-    public string Path { get; }
-    public IReadOnlyList<ConflictHunk> Hunks { get; }
+    public bool IsConflict { get; }
+    public string StableText { get; }
+    public MergeConflictBlock? Conflict { get; }
 
-    public ConflictFile(string path, IReadOnlyList<ConflictHunk> hunks)
+    private MergeFileSegment(bool isConflict, string stableText, MergeConflictBlock? conflict)
     {
-        Path = path;
-        Hunks = hunks;
+        IsConflict = isConflict;
+        StableText = stableText;
+        Conflict = conflict;
     }
 
-    public bool AllHunksResolved => Hunks.Count > 0 && Hunks.All(h => h.IsResolved);
+    public static MergeFileSegment Stable(string text) => new(false, text, null);
+    public static MergeFileSegment FromConflict(MergeConflictBlock block) => new(true, "", block);
+}
 
-    /// <summary>Merged file content: concatenation of each hunk's resolution
-    /// (single whole-file hunk today; concatenation stays correct for multi-hunk later).</summary>
-    public string ComposeResolvedContent() => string.Concat(Hunks.Select(h => h.Resolution));
+/// <summary>A conflicted file: its ordered segments (for reassembly) and just its conflict blocks.</summary>
+public sealed class MergeFile
+{
+    public string Path { get; }
+    public bool IsBinary { get; }
+    public IReadOnlyList<MergeFileSegment> Segments { get; }
+    public IReadOnlyList<MergeConflictBlock> Conflicts { get; }
+
+    public MergeFile(string path, bool isBinary, IReadOnlyList<MergeFileSegment> segments)
+    {
+        Path = path;
+        IsBinary = isBinary;
+        Segments = segments;
+        Conflicts = segments.Where(s => s.IsConflict).Select(s => s.Conflict!).ToList();
+    }
+
+    /// <summary>A binary file can't be resolved in-app; a text file is done once every block is resolved.</summary>
+    public bool AllResolved => !IsBinary && Conflicts.All(c => c.IsResolved);
+
+    /// <summary>Reassemble the file: stable text verbatim, each conflict replaced by its resolution.</summary>
+    public string Compose() => string.Concat(
+        Segments.Select(s => s.IsConflict ? (s.Conflict!.Resolution ?? s.Conflict.Ours) : s.StableText));
 }