IPv6-only for the Ethernet gateway

Split the WiFi part off common/ipv6_only.yaml into common/ipv6_only_eth.yaml
and use it on mcu-gateway. The #19750 component list needs ethernet,
otherwise the stock component still runs DHCPv4. mdns from the fork also
carries the IPv6 fix for ETH-only builds (espressif/esp-protocols#1161).

common/ipv6_only_eth.yaml

@@ -0,0 +1,30 @@
+---
+# IPv6-only IoT network (SLAAC, no DHCPv4) for ethernet devices; add next to a common package:
+#   packages:
+#     common_eth: !include common/common_eth.yaml
+#     ipv6_only: !include common/ipv6_only_eth.yaml
+# WiFi devices use common/ipv6_only.yaml (this + the IPv6 SSID)
+external_components:
+  # network enable_ipv4 + audio HTTP reader over IPv6 (esphome/esphome#19750), drop once merged
+  - source: github://pr#19750
+    components: [const, network, wifi, ethernet, audio]
+
+esp32:
+  framework:
+    components:
+      # mdns 1.12.0 + fix for "A 0.0.0.0" without IPv4 (espressif/esp-protocols#1156, merged)
+      # + IPv6 on ETH-only builds (espressif/esp-protocols#1161), drop once both are released
+      - name: espressif/mdns
+        source: https://github.com/randybb/esp-protocols.git
+        ref: mdns-v1.12.0-zero-ipv4-test
+        path: components/mdns
+      # IDF v5.5.5 lwip + .local resolution over IPv6 (espressif/esp-lwip#94), drop once in IDF
+      - name: lwip
+        source: https://github.com/randybb/esp-lwip.git
+        ref: idf-v5.5.5-lwip-component
+        path: lwip
+
+network:
+  enable_ipv6: true
+  enable_ipv4: false
+  min_ipv6_addr_count: 1