Adds Station_G3_ESP32_repeater_observer_mqtt and
Station_G3_ESP32_room_server_observer_mqtt, mirroring the known-good
Station G2 observer envs. The two boards are the same family — ESP32-S3,
qio_opi, PSRAM, 16 MB flash, SH1106 display — so the overlay ports across
unchanged: default_16MB.csv partitions, adafruit-full cert bundle,
MQTT bridge + SNMP, and the same lib_deps pinning.
Verified the G3 partition signature is byte-identical to the G2 observer's,
so the OTA partition-compatibility gate treats them the same. Both envs
build clean (1.67 MB of the 6.25 MB app slot, 28% RAM) and produce the full
CI artifact set via build.sh: .bin, -merged.bin and .partsig. Observer env
discovery now finds 32 envs (was 30) and the ArduinoJson pin check covers
both new sections.
Docs: added the G3 build commands and the two partition-table rows to
MQTT_IMPLEMENTATION.md.