common: ipv6_only package for the IPv6-only IoT network

- common/wifi.yaml: the network gets id wifi_iot, so devices can
  override it with '!extend wifi_iot'
- common/ipv6_only.yaml: add-on package next to any common package;
  switches to the IPv6-only SSID, wifi ipv6_only (SLAAC, no DHCPv4,
  esphome/esphome#19750 as external component until merged) and
  min_ipv6_addr_count: 1
- mcu-lvgl-trainer, mcu-nspanel-dev, mcu-rfid-reader, mcu-stackchan,
  mcu-tft moved to the IPv6-only network

common/ipv6_only.yaml

@@ -0,0 +1,19 @@
+---
+# IPv6-only IoT network (SLAAC, no DHCPv4); add next to a common package:
+#   packages:
+#     common: !include common/common.yaml
+#     ipv6_only: !include common/ipv6_only.yaml
+external_components:
+  # wifi ipv6_only (esphome/esphome#19750), drop once merged
+  - source: github://pr#19750
+    components: [const, network, wifi]
+
+wifi:
+  ipv6_only: true
+  networks:
+    - id: !extend wifi_iot
+      ssid: !secret wifi_ssid_ipv6
+      password: !secret wifi_password_ipv6
+
+network:
+  min_ipv6_addr_count: 1

common/wifi.yaml

@@ -2,7 +2,9 @@
 wifi:
   # reboot_timeout: 0s
   networks:
-    - ssid: !secret wifi_ssid
+    # id lets devices override the network with '!extend wifi_iot'
+    - id: wifi_iot
+      ssid: !secret wifi_ssid
       password: !secret wifi_password
   enable_btm: true
   enable_rrm: true