The geolocation shim returned a fixed Frankfurt position. With the timezone now
configurable, a deployment in another region would report a location that does
not match its exit IP or timezone, the mismatch issue #228 describes.
Set OBSCURA_GEOLOCATION="lat,lon" to override the reported coordinates; the
value is range-checked and falls back to the Frankfurt default when unset or
malformed. The existing positional jitter is preserved.