v10.1.1

fix: reject a malformed MCID instead of crashing on it

macula_content_transfer:is_chunked/2 dispatches on an MCID's two-byte
codec prefix and has no catch-all clause -- any other shape raised
FunctionClauseError in the linked worker, and in the calling process
too via get_content/2's blocking gen_server:call. Found live: a new
content-addressed thumbnail proxy passing a malformed hex string
crashed the calling process with a 500 instead of a clean error.

get_content/2, get_content_station/4,5, and macula_download's
start_link*/4,5 now validate the MCID's shape at the boundary and
return {error, invalid_mcid} instead. macula_feeder/put_content* were
never affected -- is_chunked/2's put clause dispatches on size, not
shape.