[anieto]

Why

mqtt.iata is node-wide, so every slot publishes meshcore/<same code>/…. That breaks down when an observer feeds brokers whose region maps differ. In Texas, for example, MeshMapper has LBB (Lubbock) and MAF (Midland/Odessa) but no CRP (Corpus Christi) or ACT (Waco), while MeshTexas's broker is the reverse, and it rejects codes it doesn't know. Today a Corpus Christi observer can't be correct on both: whichever code it picks is wrong (or refused) on the other network.

What

• New per-slot setting mqttN.iata <code>. Empty (the default) means "use mqtt.iata", so nothing changes for existing nodes.

  set mqtt.iata SAT     # node-wide default
  set mqtt2.iata CRP    # slot 2 publishes meshcore/CRP/...
  get mqtt2.iata        # > CRP
  set mqtt2.iata        # clear, back to mqtt.iata
  

• Applies to every MeshCore-style topic the slot publishes (presets and custom slots without a template) and to {iata} in a custom topic template. MeshRank topics carry no region code, so it has no effect there.
• Same validation as mqtt.iata (exactly 3 alphanumerics, uppercased).
• mqttSlotIata() in MQTTTopicRouter.h picks the effective code. An empty, XXX, or otherwise invalid override falls back to the global code, so a bad override can't take a working slot offline.
• Slot readiness (isSlotReady) and the set mqttN.preset hint now check the slot's effective code, so a slot with only an override counts as configured. MQTTBridge::isIATAValid() was replaced by slotIata()/isSlotIATAValid() (it had no other callers).

Persistence

• Runtime-only field MQTTPrefs::mqtt_slot_iata[6][8], appended at the end like display_timeout_secs. LegacyV1MQTTPrefs and the frozen binary layouts are untouched.
• JSON: optional iata key inside each slotN group, no format-version bump.
  • Upgrade: files without the key load with an empty override (test: PrefsWrittenBeforeSlotIataStillLoad).
  • Downgrade: older firmware ignores the unknown key, per the existing UnknownKeysInsideAKnownGroupAreDowngradeSafe behavior. The slot falls back to mqtt.iata.
• On load, a lowercase override is uppercased and an invalid one is cleared (not repaired to a default), with repaired set.

Web config

• iata added to WC_ALLOWED_SLOT_KEYS and to the /api/config slot objects.
• Slot cards get an "IATA region code" field (maxlength 3), shown for any non-none slot except token-only (MeshRank) presets. CLI autocomplete table and the mock server updated.

Tests

• test_mqtt_topic_router: override precedence/fallback; override feeds MeshCore and custom-template topics.
• test_mqtt_prefs_serializer: round trip; pre-feature file loads; case repair and invalid-override clearing.
• test_webconfig_keys: mqttN.iata is allowlisted.
• pio test -e native: all 38 suites pass. Not yet built or tested on hardware.

Notes

• The MQTT task reads the override from prefs live, the same way it already reads mqtt_slot_token. Setting it calls restartBridgeSlot() like the other slot setters.
• scripts/webconfig_cli_audit.py reports gps diag missing from the UI command table. That's pre-existing on observer-firmware-dev and unrelated to this change.