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

tests/test_diff_viewer.py

@@ -1,5 +1,5 @@
 import pytest
-from src.diff_viewer import parse_diff, DiffFile, DiffHunk, parse_hunk_header
+from src.diff_viewer import parse_diff, DiffFile, DiffHunk, parse_hunk_header, get_line_color, render_diff_text_immediate
 
 def test_parse_diff_empty() -> None:
   result = parse_diff("")
@@ -80,4 +80,25 @@ def test_diff_line_classification() -> None:
   hunk = result[0].hunks[0]
   assert any(line.startswith("-") for line in hunk.lines)
   assert any(line.startswith("+") for line in hunk.lines)
-  assert any(line.startswith(" ") or not line.startswith(("-", "+")) for line in hunk.lines)
+  assert any(line.startswith(" ") or not line.startswith(("-", "+")) for line in hunk.lines)
+
+def test_get_line_color() -> None:
+  assert get_line_color("+added") == "green"
+  assert get_line_color("-removed") == "red"
+  assert get_line_color("@@ -1,3 +1,4 @@") == "cyan"
+  assert get_line_color(" context") == None
+
+def test_render_diff_text_immediate() -> None:
+  diff_text = """--- a/test.py
++++ b/test.py
+@@ -1 +1 @@
+-old
++new"""
+  diff_files = parse_diff(diff_text)
+  output = render_diff_text_immediate(diff_files)
+  
+  assert len(output) > 0
+  assert ("File: test.py", "white") in output
+  assert ("@@ -1 +1 @@", "cyan") in output
+  assert ("-old", "red") in output
+  assert ("+new", "green") in output