Upgrade PupNet to 1.9.1 and switch to FUSE3-based AppImage runtime

[NeuralFault]

Closes #1682

What changed

Upgraded the Linux AppImage build from PupNet 1.8.0 to 1.9.1. PupNet 1.9+
no longer embeds appimagetool — it's now downloaded as a separate external
dependency. In exchange, the modern appimagetool fetches an up-to-date
type2-runtime that is statically linked against libfuse3.

Why

The old build pipeline embedded a FUSE2-based runtime that required
libfuse.so.2 on end-user systems. Modern distros (Ubuntu 24.04+, Fedora 40+)
are shipping only FUSE3, so users had to manually install libfuse2 to run the
AppImage.

The new type2-runtime:

• Is statically linked against libfuse3 — no shared library dependency
• Accepts either fusermount (libfuse2) or fusermount3 (fuse3) at runtime
• Falls back to --appimage-extract-and-run if neither is available

What end users need

Most desktop users need nothing — fuse3 is pre-installed on many modern distros.
On minimal systems: sudo apt install fuse3 (or equivalent).

Files changed

File	Change
.github/workflows/release.yml	PupNet 1.9.1, add appimagetool download, libfuse2 → fuse3
CONTRIBUTING.md	Update Linux build prerequisites
StabilityMatrix.Avalonia.pupnet.conf	Format 1.4.0 → 1.9.1, add AppImageRuntimePath
Jenkinsfile	Document new Jenkins agent prerequisites

[gemini-code-assist]

Code Review

This pull request updates the PupNet Deploy configuration to version 1.9.1, adds new configuration fields, and documents the prerequisites for publishing Linux builds in both CONTRIBUTING.md and the Jenkinsfile. The reviewer identified that appimagetool should be downloaded directly as /usr/local/bin/appimagetool rather than with the architecture suffix, as PupNet 1.9+ expects it under that exact name in the system PATH. Correspondingly, the prerequisite comments in the Jenkinsfile should be updated. Lastly, the reviewer recommended adding the -c Release flag to the pupnet command in the Jenkinsfile to ensure optimized release builds are generated.

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