fix(net): cap the length of a server line

A hostile server could stream an endless line with no newline and exhaust
the client's memory. A single reply is now capped at 16 MiB (configurable
via maxLineBytes in ConnectAsync); on excess the connection fails with
InvalidDataException. See ADR 0035.

tests/PalladiumWallet.Tests/Net/ElectrumClientTests.cs

@@ -147,6 +147,54 @@ public class ElectrumClientTests
         Assert.Equal(big, result);
     }
 
+    [Fact]
+    public async Task A_reply_just_below_the_line_limit_arrives_intact()
+    {
+        var big = new string('x', 60_000); // line = 60 KB + JSON envelope, below the 64 KB cap
+        await using var server = new FakeElectrumServer();
+        server.Handle("blockchain.transaction.get", _ => big);
+        await using var client = await ElectrumClient.ConnectAsync(server.Host, server.Port,
+            useSsl: false, maxLineBytes: 64 * 1024);
+
+        Assert.Equal(big, await client.GetTransactionAsync("00").WaitAsync(Timeout));
+        Assert.True(client.IsConnected);
+    }
+
+    [Fact]
+    public async Task A_terminated_line_above_the_limit_kills_the_connection()
+    {
+        var big = new string('x', 100_000);
+        await using var server = new FakeElectrumServer();
+        server.Handle("blockchain.transaction.get", _ => big);
+        await using var client = await ElectrumClient.ConnectAsync(server.Host, server.Port,
+            useSsl: false, maxLineBytes: 64 * 1024);
+        var disconnected = new TaskCompletionSource<Exception?>(TaskCreationOptions.RunContinuationsAsynchronously);
+        client.Disconnected += ex => disconnected.TrySetResult(ex);
+
+        await Assert.ThrowsAsync<InvalidDataException>(() => client.GetTransactionAsync("00").WaitAsync(Timeout));
+        Assert.IsType<InvalidDataException>(await disconnected.Task.WaitAsync(Timeout));
+        Assert.False(client.IsConnected);
+        await Assert.ThrowsAsync<IOException>(() => client.GetTransactionAsync("00").WaitAsync(Timeout));
+    }
+
+    [Fact]
+    public async Task An_unterminated_line_above_the_limit_fails_before_any_newline_arrives()
+    {
+        await using var server = new FakeElectrumServer();
+        server.Handle("server.banner", _ => FakeElectrumServer.NoResponse);
+        await using var client = await ElectrumClient.ConnectAsync(server.Host, server.Port,
+            useSsl: false, maxLineBytes: 64 * 1024);
+        var disconnected = new TaskCompletionSource<Exception?>(TaskCreationOptions.RunContinuationsAsynchronously);
+        client.Disconnected += ex => disconnected.TrySetResult(ex);
+
+        var pending = client.BannerAsync();
+        await server.SendUnterminatedAsync(new string('x', 100_000)); // never followed by '\n'
+
+        await Assert.ThrowsAsync<InvalidDataException>(() => pending.WaitAsync(Timeout));
+        Assert.IsType<InvalidDataException>(await disconnected.Task.WaitAsync(Timeout));
+        Assert.False(client.IsConnected);
+    }
+
     [Fact]
     public async Task A_request_without_an_answer_fails_with_TimeoutException()
     {

tests/PalladiumWallet.Tests/Net/FakeElectrumServer.cs

@@ -79,6 +79,19 @@ public sealed class FakeElectrumServer : IAsyncDisposable
             await WriteLineAsync(stream, writeLock, bytes);
     }
 
+    /// <summary>Sends raw bytes with no trailing newline (an unterminated, still-growing line).</summary>
+    public async Task SendUnterminatedAsync(string text)
+    {
+        var bytes = System.Text.Encoding.UTF8.GetBytes(text);
+        foreach (var (stream, writeLock, _) in _clients)
+        {
+            await writeLock.WaitAsync();
+            try { await stream.WriteAsync(bytes); await stream.FlushAsync(); }
+            catch (Exception) { }
+            finally { writeLock.Release(); }
+        }
+    }
+
     /// <summary>Abruptly closes every accepted connection (tests the client's failure paths).</summary>
     public void DropClients()
     {