The unknown-version path kept defaults at boot but any later savePrefs()
(every CLI set command) rewrote /mqtt_prefs as v1 with defaults, destroying
the newer config after a firmware downgrade. Latch _mqtt_prefs_hold when an
unsupported version is seen and refuse to write while it is set — checked
before the NRF52/STM32 open path, which deletes the file first.
Also pin the frozen legacy /mqtt_prefs layouts (472/1464/2904 bytes + 8-byte
header) with static_asserts so every target build re-verifies the deployed
fleet's file offsets, and null-check _obs in AlertReporter::onLoop.
agessaman
c3b8633aa7