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.

bin/gnss_simulator.rb

@@ -0,0 +1,72 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+# Speaks gpsd's own JSON wire protocol (https://gpsd.io/gpsd_json.html)
+# directly on gpsd's default port — GpsdClient (lib/maptool/gpsd_client.rb)
+# connects to it exactly as it would to real gpsd, so no app code or config
+# changes are needed. Unlike docs/development.adoc's fake-NMEA-source
+# example, this doesn't need gpsd itself installed/running at all, and
+# actually moves (a fixed point isn't much of a "GNSS position" to look
+# at) — drives a loop around a center point at a driving-ish pace, one TPV
+# report per second like a real receiver.
+#
+# Usage: bin/gnss_simulator.rb [lat] [lon] [radius_m] [port]
+require "socket"
+require "json"
+
+$stdout.sync = true
+
+LAT = (ARGV[0] || 48.1486).to_f
+LON = (ARGV[1] || 17.1077).to_f
+RADIUS_M = (ARGV[2] || 300).to_f
+PORT = (ARGV[3] || 2947).to_i
+LOOP_SECONDS = 120 # time to complete one full loop
+EARTH_RADIUS_M = 6_371_000
+
+def point_on_loop(center_lat, center_lon, radius_m, angle_rad)
+  # Flat-plane offset, not geodesic — fine at this radius (metres, not km),
+  # same reasoning as measure.js's own approximations at this scale.
+  lat_offset = (radius_m * Math.sin(angle_rad)) / EARTH_RADIUS_M
+  lon_offset = (radius_m * Math.cos(angle_rad)) / (EARTH_RADIUS_M * Math.cos(center_lat * Math::PI / 180))
+  [center_lat + lat_offset * 180 / Math::PI, center_lon + lon_offset * 180 / Math::PI]
+end
+
+server = TCPServer.new("127.0.0.1", PORT)
+puts "[gnss-simulator] driving a #{RADIUS_M.to_i}m loop around #{LAT},#{LON} — listening on 127.0.0.1:#{PORT}"
+
+loop do
+  socket = server.accept
+  puts "[gnss-simulator] client connected"
+  start = Time.now
+
+  begin
+    loop do
+      elapsed = Time.now - start
+      angle = (elapsed / LOOP_SECONDS) * 2 * Math::PI
+      lat, lon = point_on_loop(LAT, LON, RADIUS_M, angle)
+
+      # Bearing of travel: tangent to the circle, 90 degrees ahead of the
+      # radius vector at this point.
+      track = (angle * 180 / Math::PI + 90) % 360
+      speed = (2 * Math::PI * RADIUS_M) / LOOP_SECONDS # m/s, constant pace
+
+      report = {
+        class: "TPV",
+        mode: 3,
+        lat: lat,
+        lon: lon,
+        altHAE: 150.0,
+        speed: speed,
+        track: track,
+        eph: 3.0,
+        time: Time.now.utc.strftime("%Y-%m-%dT%H:%M:%S.%3NZ"),
+      }
+      socket.puts(JSON.generate(report))
+      sleep 1
+    end
+  rescue Errno::EPIPE, IOError
+    puts "[gnss-simulator] client disconnected"
+  ensure
+    socket.close
+  end
+end