Update CHANGELOG with report generation feature

CHANGELOG.md

@@ -7,6 +7,25 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- PDF Report Generation (Sprint 18)
+  - Professional PDF reports from test results with charts and styling
+  - `ReportGenerator` class orchestrating data gathering, chart generation, and PDF output
+  - `ReportConfig` for customising company name, logo, charts, and DPI
+  - HTML templates with Jinja2 for report structure
+  - CSS stylesheet optimised for A4 PDF output via WeasyPrint
+  - Matplotlib-based chart generation (voltage vs temperature, results summary)
+  - New CLI commands:
+    - `list-runs`: Display recent test runs with IDs
+    - `export-report`: Generate PDF report from run ID
+  - Dashboard PDF download button in Results Viewer
+  - Reporting configuration section in default.yaml
+  - Unit tests for models, HTML renderer, and chart generator
+  - Integration test for full report generation pipeline
+
+### Changed
+- Added `matplotlib>=3.8` to reports optional dependencies
+
 ## [0.1.0] - 2025-12-04
 
 ### Added