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.

franklinwh_agate_client/__init__.py

@@ -33,7 +33,20 @@ __all__ = [
     "HotspotProfile",
     "HotspotJoinError",
     "derive_hotspot_credentials",
+    "extract_soc",
+    "get_battery",
+    "SOCKET_PORT_CANDIDATES",
     "DEVICE_GATEWAY_IP",
     "FALLBACK_PASSWORD",
     "VALID_SN_LENGTH",
 ]
+
+
+def __getattr__(name: str):
+    # Lazy so `python -m franklinwh_agate_client.battery` does not import
+    # the submodule twice (avoids a runpy RuntimeWarning).
+    if name in {"extract_soc", "get_battery", "SOCKET_PORT_CANDIDATES"}:
+        from . import battery as _battery
+
+        return getattr(_battery, name)
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")