Minimal Set-Cookie parser. We accept the subset the PX servers
emit — name=value; Domain=…; Path=…; …. We do NOT honour
Expires/Max-Age here; cookie freshness is the caller’s concern via
PxCookieBundle::expires_at.
Native NativeSolver implementation that builds a sensor payload
locally and POSTs it to ${origin}/${app_id_tag}${sensor_path},
then parses the Set-Cookie response into a PxCookieBundle.