Fix safe library output handling

[perNyfelt]

Summary

• Render PDF files to a temporary sibling and replace the target only after a successful render.
• Make OpenHTMLtoPDF SLF4J logging an explicit application-startup choice, while preserving the GUI's current logging behavior.
• Make image MIME inference locale-independent and strengthen concurrency test failure reporting.
• Add 0.2.0 release-note sections for the parent, runtime, and GUI artifacts.

Root cause

Renderer.toPdf(File) opened the final destination before rendering, truncating a previously generated PDF if rendering failed. Constructing an engine also replaced the process-wide OpenHTMLtoPDF logger unexpectedly.

Validation

mvn spotless:apply && mvn verify