test: add in-process fake ElectrumX server for network-layer testing

Real loopback TCP socket speaking newline-delimited JSON-RPC 2.0, optionally
TLS with a self-signed certificate, with per-method handlers and call
counters. Lets ElectrumClient, WalletSynchronizer, TransactionInspector, and
CertificatePinStore be exercised against the same code paths used in
production instead of being mocked.

Also exposes UpdateChecker's tag-parsing logic internally so its version
comparison can be tested without a real GitHub API call.

src/Core/Net/UpdateChecker.cs

@@ -55,7 +55,7 @@ public static class UpdateChecker
     }
 
     /// <summary>Parses "v1.2.3" / "1.2.3-beta" style tags into a comparable <see cref="Version"/>.</summary>
-    private static bool TryParse(string raw, out Version version)
+    internal static bool TryParse(string raw, out Version version)
     {
         var s = raw.Trim();
         if (s.StartsWith('v') || s.StartsWith('V')) s = s[1..];