feat(examples): add Ethernet sublayer example and update PTP

Add a dedicated sublayer example under examples/ethernet and enable
sublayer usage in the PTP example.

examples/ethernet/sublayer/pytest_eth_sublayer.py

@@ -0,0 +1,26 @@
+# SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+# SPDX-License-Identifier: Unlicense OR CC0-1.0
+import platform
+import subprocess
+
+import pytest
+from pytest_embedded import Dut
+
+
+@pytest.mark.parametrize(
+    'config, target',
+    [
+        pytest.param('defaults', 'esp32', marks=[pytest.mark.eth_ip101]),
+        pytest.param('defaults_esp32p4', 'esp32p4', marks=[pytest.mark.eth_ip101]),
+        pytest.param('defaults_esp32s31', 'esp32s31', marks=[pytest.mark.eth_yt8531]),
+    ],
+    indirect=['target'],
+)
+def test_esp_eth_sublayer(dut: Dut) -> None:
+    # wait for IP on the untagged interface
+    dut_ip = dut.expect(r'esp_netif_handlers: .+ ip: (\d+\.\d+\.\d+\.\d+),').group(1)
+    param = '-n' if platform.system().lower() == 'windows' else '-c'
+    command = ['ping', param, '1', dut_ip]
+    output = subprocess.run(command, capture_output=True)
+    if 'unreachable' in str(output.stdout):
+        raise RuntimeError('Host unreachable')