Behaviour for supervised content downloads (the get/fetch side).

get_content/2 is a plain blocking call — no addressable pid to cancel it from outside. This is the consumer-side counterpart to macula_feeder: start_link/4,5 returns immediately with a pid, runs macula:get_content/2 in a linked worker, delivers the outcome to Module:handle_downloaded/2, and publishes sharing.get_started_v1 / sharing.get_completed_v1 mesh facts around the transfer — including outcome => cancelled if the download is cancelled before the get resolves.

This is content sharing, not general-purpose RPC streaming — see macula_streamer / macula_stream_sink for that (streaming.* facts belong to that pair).

Example

   -module(doc_download).
   -behaviour(macula_download).
   -export([init/1, handle_downloaded/2]).
  
   init(Parent) -> {ok, Parent}.
  
   handle_downloaded(Result, Parent) ->
       Parent ! {downloaded, Result},
       {stop, normal, Parent}.

   {ok, Pid} = macula_download:start_link(doc_download, Pool, Realm,
       Mcid, self()).