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>

tests/unit/test_electrum_client.py

@@ -325,12 +325,30 @@ class _PinWriter(FakeWriter):
         self.closed = True
 
 
+def _answer_features(reader, writer, result=None):
+    """Make `writer` answer `server.features` the moment it is asked: a reply fed up front
+    would be read before its request is registered, and dropped."""
+    from app.wallet.plm_network import GENESIS_HASH
+
+    original = writer.write
+
+    def write(data):
+        original(data)
+        sent = json.loads(data.decode())
+        if sent["method"] == "server.features":
+            reply = {"genesis_hash": GENESIS_HASH} if result is None else result
+            reader.feed_data((json.dumps({"id": sent["id"], "result": reply}) + "\n").encode())
+
+    writer.write = write
+
+
 async def _connect_with_pin(monkeypatch, pin, der):
     import hashlib  # noqa: F401
 
     writer = _PinWriter(der)
     reader = asyncio.StreamReader()
     reader.feed_data(b'{"id": 1, "result": ["fake", "1.4"]}\n')
+    _answer_features(reader, writer)
 
     async def fake_open_connection(host, port, **kwargs):
         return reader, writer
@@ -413,3 +431,22 @@ async def test_only_a_server_error_object_is_an_explicit_refusal():
         silent._read_task.cancel()
     finally:
         client_module._REQUEST_TIMEOUT_SECONDS = original
+
+
+async def test_a_server_on_another_chain_is_refused_at_the_handshake(monkeypatch):
+    """ELC-30: Bitcoin's genesis (or no genesis at all) fails like an unreachable server."""
+    import pytest
+
+    bitcoin = "000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26e"
+    for result in ({"genesis_hash": bitcoin}, {}):
+        writer = FakeWriter()
+        reader = asyncio.StreamReader()
+        reader.feed_data(b'{"id": 1, "result": ["fake", "1.4"]}\n')
+        _answer_features(reader, writer, result)
+
+        async def fake_open_connection(host, port, reader=reader, writer=writer, **kwargs):
+            return reader, writer
+
+        monkeypatch.setattr(asyncio, "open_connection", fake_open_connection)
+        with pytest.raises(ElectrumError, match="not on the PLM chain"):
+            await ElectrumClient("localhost", 1).connect()