Named constants for MQTT reason codes, so protocol code reads by name instead of hex literal.

Covers the MQTT 5.0 reason codes (§2.4) and MQTT 3.1.1 CONNACK return codes (§3.2.2.3) this library sends. The full v5 table lives in the spec; add codes here as they gain call sites rather than mirroring the whole table (an earlier complete-table module ended up as dead code).