feat(diff): Add diff rendering helpers for GUI

- Add get_line_color() to classify diff lines
- Add render_diff_text_immediate() for immediate mode rendering
- Add tests for rendering functions

src/diff_viewer.py

@@ -126,4 +126,24 @@ def format_diff_for_display(diff_files: List[DiffFile]) -> str:
       output.append(f"  {hunk.header}")
       for line in hunk.lines:
         output.append(f"    {line}")
-  return "\n".join(output)
+  return "\n".join(output)
+
+def get_line_color(line: str) -> Optional[str]:
+  if line.startswith("+"):
+    return "green"
+  elif line.startswith("-"):
+    return "red"
+  elif line.startswith("@@"):
+    return "cyan"
+  return None
+
+def render_diff_text_immediate(diff_files: List[DiffFile]) -> List[tuple[str, Optional[str]]]:
+  output: List[tuple[str, Optional[str]]] = []
+  for df in diff_files:
+    output.append((f"File: {df.old_path}", "white"))
+    for hunk in df.hunks:
+      output.append((hunk.header, "cyan"))
+      for line in hunk.lines:
+        color = get_line_color(line)
+        output.append((line, color))
+  return output