A 42-year-old woman with a history of diverticular disease presents with acute left lower quadrant pain, fever (39.2°C), and leukocytosis (WBC 16,000/μL). On examination, she has localized peritonitis in the left iliac fossa. CT abdomen/pelvis with IV contrast shows a thickened sigmoid colon with adjacent fat stranding, a small collection (3 cm) in the pelvis, and a **thin linear lucency tracking through the sigmoid wall into the peritoneal cavity**. What is the most likely diagnosis?