emqtt's transitive `quicer` dep needs CMake + msquic submodule
clones to build, which the slim Debian builder image doesn't ship —
the prod CI build was failing inside `mix deps.compile`. Tortoise311
is the obvious pure-Elixir alternative but it bundles a
gen_state_machine + retry/inflight stack we don't use because we're
subscribe-only at QoS 0.
Implementing the wire protocol inline is ~150 lines in
`Microwaveprop.Pskr.Mqtt` (CONNECT/SUBSCRIBE/PINGREQ/DISCONNECT
builders, CONNACK/PUBLISH/SUBACK/PINGRESP parser, varint codec).
Pskr.Client now uses :gen_tcp with `active: :once` framing,
buffers partial reads, schedules its own keepalive, and parses
inbound packets in a tight loop.
Drops `BUILD_WITHOUT_QUIC=1` from the Dockerfile — no longer
relevant since there's no native dep at all in the build chain
now.
