Clean up repository and update to .NET 10 LTS

- Remove documentation files (ANALYZER.md, ANALYZER_DISCOVERY.md, CLAUDE.md)
- Remove 126 test-generated PDF files from version control
- Remove build-output.txt (build logs)
- Update .gitignore to exclude test outputs, coverage results, and temp files
- Update Dockerfile to use stable .NET 10 SDK instead of preview
- Remove 'preview' tag from package metadata
- Add RuntimeSystemEnvironmentTests.cs for system environment testing
- Fix TextRenderer null reference handling
- Add edge case tests for TextRenderer

Author: ANcpLua

.gitignore

@@ -148,4 +148,16 @@ ehthumbs.db
 TestResult.xml
 nunit-*.xml
 
-ocr/
+ocr/
+
+# Test outputs
+CreatePdf.NET.Tests/output/
+test_output.txt
+build-output.txt
+
+# Coverage results
+coverage-results/
+coverage-check/
+
+# Temporary files
+temp/