Add SoftAP battery PoC (apower/BMS/EMS socket read) with docs and tests

Probes 10.100.1.1:9000,18000, sends 0x19/0x1D/0x1E envelopes, extracts
currentBatterySoc/soc/avg(socArray). Includes offline extract_soc tests,
BATTERY_POC docs, README usage, and empirical port-9000 RE notes.

tests/test_battery.py

@@ -0,0 +1,39 @@
+"""Offline tests for battery SoC extraction (no radio/hardware needed)."""
+
+import unittest
+
+from franklinwh_agate_client.battery import extract_soc
+
+
+class TestExtractSoc(unittest.TestCase):
+    def test_current_battery_soc_wins(self):
+        reply = {"cmdType": 25, "data": {"currentBatterySoc": 78, "soc": 10}}
+        out = extract_soc(reply)
+        self.assertEqual(out["value"], 78)
+        self.assertEqual(out["source"], "currentBatterySoc")
+
+    def test_soc_fallback(self):
+        out = extract_soc({"data": {"soc": "55"}})
+        self.assertEqual(out["value"], 55)
+        self.assertEqual(out["source"], "soc")
+
+    def test_soc_array_averaged(self):
+        out = extract_soc({"data": {"socArray": [80, 90]}})
+        self.assertEqual(out["value"], 85)
+        self.assertEqual(out["source"], "socArray(avg)")
+
+    def test_nested_data_string(self):
+        import json
+
+        reply = {"data": json.dumps({"aPower": {"soc": 62}})}
+        out = extract_soc(reply)
+        self.assertEqual(out["value"], 62)
+
+    def test_missing_soc(self):
+        out = extract_soc({"data": {"foo": 1}})
+        self.assertIsNone(out["value"])
+        self.assertIsNone(out["source"])
+
+
+if __name__ == "__main__":
+    unittest.main()