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.

docs/development.adoc

@@ -52,9 +52,28 @@ up as). On openSUSE, `gpsd` is a package with systemd units
 package manager. No app-side configuration is needed — it just
 connects to `127.0.0.1:2947`, gpsd's default port.
 
-To test without hardware, feed gpsd a fake NMEA stream over TCP
-instead of a real serial device — gpsd accepts a `tcp://host:port`
-source:
+To test without hardware or gpsd itself installed/running, run
+`bin/gnss_simulator.rb` — it speaks gpsd's own JSON wire protocol
+directly on gpsd's default port (`127.0.0.1:2947`), so
+`Maptool::GpsdClient` connects to it exactly as it would to real
+gpsd, no app-side changes needed. Drives a loop around a center point
+(Bratislava by default) at a driving-ish pace instead of sitting at a
+fixed position, since a stationary "GNSS position" isn't much to look
+at on the map:
+
+[source,shell]
+----
+bin/gnss_simulator.rb                       # loop around Bratislava, 300m radius
+bin/gnss_simulator.rb 48.85 2.35 500         # custom center/radius (Paris, 500m)
+----
+
+`GET /api/gnss/position` should report a fix within a couple of
+seconds, moving on every poll.
+
+To test against gpsd itself instead (e.g. to exercise the actual
+NMEA-parsing path, not just gpsd's JSON output), feed it a fake NMEA
+stream over TCP instead of a real serial device — gpsd accepts a
+`tcp://host:port` source:
 
 [source,shell]
 ----