snap: pin qet-tb-generator source to tag v1.31

Without a source-tag or source-commit, snapcraft pulls the latest
HEAD of qet_tb_generator-plugin on every build.  This makes builds
non-reproducible and risks breakage whenever the upstream repo changes.

Pin to the only published release tag (v1.31, commit d6ee3cf) so
the snap always builds against a known-good version of the plugin.

Closes #202

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

build-aux/snap/snapcraft.yaml

@@ -53,6 +53,7 @@ parts:
   qet-tb-generator:
     plugin: python
     source: https://github.com/raulroda/qet_tb_generator-plugin.git
+    source-tag: v1.31
     python-packages: [PySimpleGUI]
     stage-packages: 
       - python3-lxml