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

src/diff_viewer.py

@@ -0,0 +1,129 @@
+from typing import List, Dict, Optional
+from dataclasses import dataclass
+
+@dataclass
+class DiffHunk:
+  header: str
+  lines: List[str]
+  old_start: int
+  old_count: int
+  new_start: int
+  new_count: int
+
+@dataclass  
+class DiffFile:
+  old_path: str
+  new_path: str
+  hunks: List[DiffHunk]
+
+def parse_diff_header(line: str) -> tuple[Optional[str], Optional[str], Optional[tuple[int, int, int, int]]]:
+  if not line.startswith(("--- ", "+++ ")):
+    return None, None, None
+  
+  if line.startswith("--- "):
+    path = line[4:]
+    if path.startswith("a/"):
+      path = path[2:]
+    return path, None, None
+  elif line.startswith("+++ "):
+    path = line[4:]
+    if path.startswith("b/"):
+      path = path[2:]
+    return None, path, None
+  return None, None, None
+
+def parse_hunk_header(line: str) -> Optional[tuple[int, int, int, int]]:
+  if not line.startswith("@@"):
+    return None
+  
+  parts = line.split()
+  if len(parts) < 2:
+    return None
+  
+  old_part = parts[1][1:]
+  new_part = parts[2][1:]
+  
+  old_parts = old_part.split(",")
+  new_parts = new_part.split(",")
+  
+  old_start = int(old_parts[0])
+  old_count = int(old_parts[1]) if len(old_parts) > 1 else 1
+  new_start = int(new_parts[0])
+  new_count = int(new_parts[1]) if len(new_parts) > 1 else 1
+  
+  return (old_start, old_count, new_start, new_count)
+
+def parse_diff(diff_text: str) -> List[DiffFile]:
+  if not diff_text or not diff_text.strip():
+    return []
+  
+  files: List[DiffFile] = []
+  current_file: Optional[DiffFile] = None
+  current_hunk: Optional[DiffHunk] = None
+  
+  for line in diff_text.split("\n"):
+    if line.startswith("--- "):
+      if current_file:
+        if current_hunk:
+          current_file.hunks.append(current_hunk)
+          current_hunk = None
+        files.append(current_file)
+      
+      path = line[4:]
+      if path.startswith("a/"):
+        path = path[2:]
+      current_file = DiffFile(old_path=path, new_path="", hunks=[])
+      
+    elif line.startswith("+++ ") and current_file:
+      path = line[4:]
+      if path.startswith("b/"):
+        path = path[2:]
+      current_file.new_path = path
+      
+    elif line.startswith("@@") and current_file:
+      if current_hunk:
+        current_file.hunks.append(current_hunk)
+      
+      hunk_info = parse_hunk_header(line)
+      if hunk_info:
+        old_start, old_count, new_start, new_count = hunk_info
+        current_hunk = DiffHunk(
+          header=line,
+          lines=[],
+          old_start=old_start,
+          old_count=old_count,
+          new_start=new_start,
+          new_count=new_count
+        )
+      else:
+        current_hunk = DiffHunk(
+          header=line,
+          lines=[],
+          old_start=0,
+          old_count=0,
+          new_start=0,
+          new_count=0
+        )
+        
+    elif current_hunk is not None:
+      current_hunk.lines.append(line)
+      
+    elif line and not line.startswith("diff ") and not line.startswith("index "):
+      pass
+  
+  if current_file:
+    if current_hunk:
+      current_file.hunks.append(current_hunk)
+    files.append(current_file)
+  
+  return files
+
+def format_diff_for_display(diff_files: List[DiffFile]) -> str:
+  output = []
+  for df in diff_files:
+    output.append(f"File: {df.old_path}")
+    for hunk in df.hunks:
+      output.append(f"  {hunk.header}")
+      for line in hunk.lines:
+        output.append(f"    {line}")
+  return "\n".join(output)