test: add ServerProbe tool and opt-in live server test

ServerProbe is a read-only conformance/health checker for Electrum
protocol servers (Fulcrum, ElectrumX): handshake/TLS, features and
genesis, header linkage, Merkle proof vs header root, fees, latency.
LiveServerTests runs it from dotnet test when PALLADIUM_TEST_SERVERS is
set. Also ignore .claude/, apk/aab and local.properties.

tests/PalladiumWallet.Tests/Net/LiveServerTests.cs

@@ -0,0 +1,45 @@
+using System.Reflection;
+
+namespace PalladiumWallet.Tests.Net;
+
+/// <summary>
+/// Opt-in conformance run of the ServerProbe against real Electrum/Fulcrum servers.
+/// Does nothing (passes) unless <c>PALLADIUM_TEST_SERVERS</c> is set, so the default
+/// <c>dotnet test</c> stays offline and deterministic.
+///
+///   PALLADIUM_TEST_SERVERS=host1:50002,host2:50002   servers to probe (comma separated)
+///   PALLADIUM_TEST_SSL=1                              use TLS
+///   PALLADIUM_TEST_NET=mainnet|testnet|regtest        network (default mainnet)
+///
+/// Example: PALLADIUM_TEST_SERVERS=1.2.3.4:50002 PALLADIUM_TEST_SSL=1 dotnet test --filter LiveServer
+/// </summary>
+[Collection("Console")] // the probe writes to Console, which is process-global
+public class LiveServerTests
+{
+    [Fact]
+    public async Task LiveServer_PassesProbe()
+    {
+        var servers = Environment.GetEnvironmentVariable("PALLADIUM_TEST_SERVERS");
+        if (string.IsNullOrWhiteSpace(servers)) return;
+
+        var args = new List<string>();
+        foreach (var s in servers.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries))
+            args.AddRange(["--server", s]);
+        if (Environment.GetEnvironmentVariable("PALLADIUM_TEST_SSL") is "1" or "true") args.Add("--ssl");
+        if (Environment.GetEnvironmentVariable("PALLADIUM_TEST_NET") is { Length: > 0 } net) args.AddRange(["--net", net]);
+
+        // The probe is a top-level-statements exe: call its entry point rather than shelling out.
+        var entry = Assembly.Load("PalladiumWallet.ServerProbe").EntryPoint
+            ?? throw new InvalidOperationException("ServerProbe has no entry point.");
+
+        var original = Console.Out;
+        var captured = new StringWriter();
+        Console.SetOut(captured);
+        int exit;
+        // The synthesized entry point is a synchronous Main(string[]) → int wrapper.
+        try { exit = await Task.Run(() => (int)entry.Invoke(null, [args.ToArray()])!); }
+        finally { Console.SetOut(original); }
+
+        Assert.True(exit == 0, captured.ToString());
+    }
+}