unchanged() ignores ZMQ reads in Julia, so the read loop never exits #628

Same thing as #612 but in Julia. A ZMQ read in concore.jl never updates s, so unchanged() stays true even after real data comes in. A while unchanged(); concore_read(...) loop on a ZMQ port never exits.

File reads do s = _cap_s(s, ins) (concore.jl line 710), but _zmq_read() (line 272) returns without touching s. concoredocker.jl has the same code (line 273).

init_zmq_port("out", "bind", "tcp://127.0.0.1:5571", "PAIR")
init_zmq_port("in", "connect", "tcp://127.0.0.1:5571", "PAIR")
concore_write("out", "u", [10.0])
unchanged()
concore_read("in", "u", "[0.0, 0.0]")   # [10.0]
unchanged()                             # still true

Expected: false after a successful ZMQ read, like file ports. Same steps on file port 1 give false.

Tested on dev at 8f3e444, Julia 1.11.7, ZMQ.jl 1.5.1