feat(diff): Add diff parser for unified diff format

- Create src/diff_viewer.py with parse_diff function
- Parse unified diff into DiffFile and DiffHunk dataclasses
- Extract file paths, hunk headers, and line changes
- Add unit tests for diff parser

tests/test_diff_viewer.py

@@ -0,0 +1,83 @@
+import pytest
+from src.diff_viewer import parse_diff, DiffFile, DiffHunk, parse_hunk_header
+
+def test_parse_diff_empty() -> None:
+  result = parse_diff("")
+  assert result == []
+
+def test_parse_diff_none() -> None:
+  result = parse_diff(None)  # type: ignore
+  assert result == []
+
+def test_parse_simple_diff() -> None:
+  diff_text = """--- a/src/test.py
++++ b/src/test.py
+@@ -1 +1 @@
+-old
++new"""
+  result = parse_diff(diff_text)
+  assert len(result) == 1
+  assert result[0].old_path == "src/test.py"
+  assert result[0].new_path == "src/test.py"
+  assert len(result[0].hunks) == 1
+  assert result[0].hunks[0].header == "@@ -1 +1 @@"
+
+def test_parse_diff_with_context() -> None:
+  diff_text = """--- a/src/example.py
++++ b/src/example.py
+@@ -10,5 +10,6 @@
+ def existing_function():
+     pass
+-    old_line
++    old_line
++    new_line
+     more_code"""
+  result = parse_diff(diff_text)
+  assert len(result) == 1
+  assert result[0].old_path == "src/example.py"
+  assert len(result[0].hunks) == 1
+  hunk = result[0].hunks[0]
+  assert hunk.old_start == 10
+  assert hunk.old_count == 5
+  assert hunk.new_start == 10
+  assert hunk.new_count == 6
+  assert "-    old_line" in hunk.lines
+  assert "+    new_line" in hunk.lines
+
+def test_parse_multiple_files() -> None:
+  diff_text = """--- a/file1.py
++++ b/file1.py
+@@ -1 +1 @@
+-a
++b
+--- a/file2.py
++++ b/file2.py
+@@ -1 +1 @@
+-c
++d"""
+  result = parse_diff(diff_text)
+  assert len(result) == 2
+  assert result[0].old_path == "file1.py"
+  assert result[1].old_path == "file2.py"
+
+def test_parse_hunk_header() -> None:
+  result = parse_hunk_header("@@ -10,5 +10,6 @@")
+  assert result == (10, 5, 10, 6)
+  
+  result = parse_hunk_header("@@ -1 +1 @@")
+  assert result == (1, 1, 1, 1)
+
+def test_diff_line_classification() -> None:
+  diff_text = """--- a/test.py
++++ b/test.py
+@@ -1,3 +1,4 @@
+ context line
+-removed line
++removed line
++added line
+ another context"""
+  result = parse_diff(diff_text)
+  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)