Refuse an Electrum server that is not on the PLM chain (B-99, ELC-30)

PLM shares Bitcoin's genesis and powLimit, so a Bitcoin server passed every
header check. connect() now asks server.features and requires PLM's genesis.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

app/electrum/client.py

@@ -7,6 +7,8 @@ import re
 import ssl
 from typing import NamedTuple
 
+from app.wallet.plm_network import GENESIS_HASH
+
 
 logger = logging.getLogger(__name__)
 
@@ -192,6 +194,20 @@ class ElectrumClient:
                 raise
         self._read_task = asyncio.create_task(self._read_loop())
         await self.request("server.version", ["plm-lottery", "1.4"])
+        try:
+            await self._check_genesis()
+        except ElectrumError:
+            self._read_task.cancel()
+            self._writer.close()
+            raise
+
+    async def _check_genesis(self) -> None:
+        """The server must be on PLM's chain, not just any chain with valid headers: PLM shares
+        Bitcoin's genesis and powLimit, so only `server.features` tells them apart (B-99, ELC-30)."""
+        features = await self.request("server.features")
+        genesis = features.get("genesis_hash") if isinstance(features, dict) else None
+        if genesis != GENESIS_HASH:
+            raise ElectrumError(f"{self.host}:{self.port} is not on the PLM chain (genesis {genesis!r})")
 
     def _check_pin(self) -> None:
         """The server's DER certificate must hash to the pinned SHA-256 (B-84, ELC-03)."""