Add a GNSS simulator for dev without gpsd running

bin/gnss_simulator.rb speaks gpsd's own JSON wire protocol directly
on its default port -- GpsdClient connects to it exactly as it would
to real gpsd, no app changes needed, and no gpsd install/daemon
required either (unlike the existing fake-NMEA-source approach in
docs/development.adoc, which still needs gpsd itself in front of it).
Drives a loop around a center point at a driving-ish pace rather than
sitting still, since a fixed position isn't much of a "GNSS position"
to look at.

Verified end to end: started it, pointed a dev server at it, and
confirmed /api/gnss/position reports a moving fix across two polls.

.gitea/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
           bundler-cache: true
 
       - name: Ruby syntax check
-        run: find app.rb config lib -name "*.rb" -print0 | xargs -0 -n1 ruby -c
+        run: find app.rb config lib bin -name "*.rb" -print0 | xargs -0 -n1 ruby -c
 
       - name: App loads without errors
         run: bundle exec ruby -e 'require_relative "app"'