test: cover network/SPV layer end to end (was previously untested)

ElectrumClient, WalletSynchronizer, TransactionInspector, CertificatePinStore
and peer discovery went from 0% coverage to close to full coverage, all
against the fake ElectrumX server:

- ElectrumClient: request/response correlation under load, server errors,
  notifications, disconnection, cancellation, SSL handshake with TOFU pinning.
- WalletSynchronizer: gap-limit scanning, UTXO/history reconstruction,
  unconfirmed/immature balances, busy-retry, disk-cache reuse, and the
  security-critical path where a lying server fails Merkle verification and
  the sync must abort.
- TransactionInspector: fee calculation from resolved inputs, mine/theirs
  attribution, RBF flag, coinbase handling, degraded paths when a previous
  transaction can't be resolved.
- CertificatePinStore: TOFU pin/match/mismatch/reset, corrupted-file fallback.
- ServerRegistry: peer discovery and persistence via DiscoverAsync.
- UpdateChecker: release-tag parsing and semver comparison.

tests/PalladiumWallet.Tests/Net/UpdateCheckerTests.cs

@@ -0,0 +1,44 @@
+using PalladiumWallet.Core.Net;
+
+namespace PalladiumWallet.Tests.Net;
+
+/// <summary>
+/// Tests for the release-tag parsing used by the update check. The network
+/// call itself is best-effort by design (any failure → null) and is not
+/// exercised here: only the version-comparison logic is deterministic.
+/// </summary>
+public class UpdateCheckerTests
+{
+    [Theory]
+    [InlineData("v1.2.3", "1.2.3")]
+    [InlineData("V0.9.1", "0.9.1")]
+    [InlineData("1.2.3", "1.2.3")]
+    [InlineData("0.9.0-beta", "0.9.0")]
+    [InlineData("v2.0.0-rc.1", "2.0.0")]
+    [InlineData(" 1.4 ", "1.4")]
+    public void I_tag_di_release_si_parsano_nella_versione_attesa(string tag, string expected)
+    {
+        Assert.True(UpdateChecker.TryParse(tag, out var version));
+        Assert.Equal(Version.Parse(expected), version);
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("main")]
+    [InlineData("v")]
+    [InlineData("1")]
+    [InlineData("non-una-versione")]
+    public void I_tag_non_versionati_vengono_rifiutati(string tag)
+    {
+        Assert.False(UpdateChecker.TryParse(tag, out _));
+    }
+
+    [Fact]
+    public void Il_confronto_remota_maggiore_di_locale_segue_l_ordine_semver()
+    {
+        // The same comparison CheckAsync applies between remote tag and running app.
+        Assert.True(UpdateChecker.TryParse("v0.10.0", out var remote));
+        Assert.True(UpdateChecker.TryParse("0.9.0", out var local));
+        Assert.True(remote > local); // 0.10 > 0.9: numeric, not lexicographic
+    }
+}