feat: add Java JRXML-to-PNG rendering pipeline with pixel-level SSIM comparison

- lib/java/: Java renderer (JrxmlRenderer) using JasperReports 6.21.0
  - JrxmlDebug for diagnostics, JrxmlGen for format reference
  - download_jars.sh for one-time dependency setup
- agent/nodes.py: _render_jrxml_to_png() and _compute_pixel_similarity()
  - Pixel comparison integrates into validate node (SSIM < 0.4 fails)
  - Pixel fidelity context injected into correct_jrxml for targeted fixes
- tests/test_pixel_comparison.py: 15 unit tests (render, SSIM, integration)
- .gitignore: exclude lib/java/*.jar, lib/java/*.class, tmp/
- CLAUDE.md: v11 changelog documenting the rendering pipeline
- All non-LLM tests pass (97/97)

.gitignore

@@ -20,6 +20,13 @@ EVALUATION_REPORT.md
 # 上传文件
 uploads/
 
+# Java JARs & compiled classes
+lib/java/*.jar
+lib/java/*.class
+
+# 渲染临时文件
+tmp/
+
 # OCR 临时输出
 ocr_raw_positions.json