feat(ui): add StreamLineFormatter for NDJSON stream parsing

Parses Claude CLI stream-json output into human-readable text.
Handles text deltas, tool use, results, API retries, and trims large files.
All 13 tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Helpers/StreamLineFormatter.cs

@@ -0,0 +1,115 @@
+using System.Text;
+using System.Text.Json;
+
+namespace ClaudeDo.Ui.Helpers;
+
+public class StreamLineFormatter
+{
+    private const int MaxLength = 50_000;
+
+    public string? FormatLine(string line)
+    {
+        JsonDocument doc;
+        try
+        {
+            doc = JsonDocument.Parse(line);
+        }
+        catch (JsonException)
+        {
+            return line;
+        }
+
+        using (doc)
+        {
+            var root = doc.RootElement;
+            if (!root.TryGetProperty("type", out var typeProp))
+                return null;
+
+            var type = typeProp.GetString();
+
+            switch (type)
+            {
+                case "stream_event":
+                    return FormatStreamEvent(root);
+
+                case "result":
+                    if (root.TryGetProperty("result", out var resultProp))
+                        return $"\n--- Result ---\n{resultProp.GetString()}\n";
+                    return null;
+
+                case "system":
+                    if (root.TryGetProperty("subtype", out var subtypeProp) &&
+                        subtypeProp.GetString() == "api_retry")
+                        return "\n[Retrying API call...]\n";
+                    return null;
+
+                default:
+                    return null;
+            }
+        }
+    }
+
+    private static string? FormatStreamEvent(JsonElement root)
+    {
+        if (!root.TryGetProperty("event", out var ev))
+            return null;
+        if (!ev.TryGetProperty("type", out var evTypeProp))
+            return null;
+
+        var evType = evTypeProp.GetString();
+
+        switch (evType)
+        {
+            case "content_block_delta":
+                if (!ev.TryGetProperty("delta", out var delta))
+                    return null;
+                if (!delta.TryGetProperty("type", out var deltaTypeProp))
+                    return null;
+                var deltaType = deltaTypeProp.GetString();
+                if (deltaType == "text_delta")
+                {
+                    return delta.TryGetProperty("text", out var textProp)
+                        ? textProp.GetString()
+                        : null;
+                }
+                return null; // input_json_delta and others → skip
+
+            case "content_block_stop":
+                return "\n";
+
+            case "content_block_start":
+                if (!ev.TryGetProperty("content_block", out var cb))
+                    return null;
+                if (cb.TryGetProperty("type", out var cbTypeProp) &&
+                    cbTypeProp.GetString() == "tool_use" &&
+                    cb.TryGetProperty("name", out var nameProp))
+                    return $"\n[Tool: {nameProp.GetString()}]\n";
+                return null;
+
+            default:
+                return null; // message_start, message_delta, etc.
+        }
+    }
+
+    public string FormatFile(string filePath)
+    {
+        var sb = new StringBuilder();
+        foreach (var line in File.ReadLines(filePath))
+        {
+            var formatted = FormatLine(line);
+            if (formatted is not null)
+                sb.Append(formatted);
+        }
+        return Trim(sb.ToString());
+    }
+
+    public static string Trim(string text)
+    {
+        if (text.Length <= MaxLength) return text;
+        var trimStart = text.Length - MaxLength;
+        var newlineAfter = text.IndexOf('\n', trimStart);
+        if (newlineAfter >= 0 && newlineAfter < trimStart + 200)
+            trimStart = newlineAfter + 1;
+        return text[trimStart..];
+    }
+}