Heltec V4 legacy-partition migration over Wi-Fi

Heltec V4/V4.3 OLED repeaters with their normal 16 MiB flash use the shared ESP32 partition migration bridge. It changes Arduino's old default.csv layout, with two 1.25 MiB OTA slots, to default_16MB.csv, with two 6.25 MiB OTA slots.

Build the complete V4 package with the repeatable ESP32 recipe in WSL/Linux:

python3 -B scripts/build_esp32_partition_migration.py \
  --board heltec-v4 --version v1.17.1.7-halo-keymind-cascade-dev \
  --radio-preset usa-cascadia --profile cascade

The ZIP contains the Wi-Fi bridge, LoRa bridge, and exact-target Full image. Upload its wifi-bridge.bin through the legacy browser Wi-Fi updater. Rejoin MeshCore-Migrate with password meshcore-migrate; when it reports Expanded layout ready, upload the ZIP's full-application.bin at /update.

Do not use an -merged.bin in either browser uploader. Merged images contain bootloader and table offsets for cable flashing. The bridge preserves the private identity; other SPIFFS-backed settings may be recreated after the Full image starts.

For an existing heltec_v4_repeater that already supports LoRa mOTA, use the separate LoRa migration package described in the shared guide. It preserves the old repeater across the partition change so the full image can also be installed over LoRa without reaching the board's Wi-Fi network.