feat(fuzz): add SharpFuzz-based fuzzing for untrusted-input parsers

New tests/PalladiumWallet.Fuzz project: one target per parser that
consumes untrusted input (block headers, Merkle proofs, peer lists,
wallet files, user-pasted mnemonics/keys/addresses/amounts), each
encoding the parser's documented error contract - any exception beyond
that contract is a finding, reported as a crash.

Three ways to run: the seed corpus (with regression inputs for every
crash found so far, including the two fixed in prior commits) replays
automatically inside dotnet test via FuzzCorpusTests, so a fixed
contract violation can't silently return; a built-in random-mutation
mode needs no external tooling (dotnet run -- <target> --random N);
and fuzz.sh drives real coverage-guided campaigns via afl++ +
SharpFuzz instrumentation for pre-release or post-parser-change runs.

tests/PalladiumWallet.Tests/FuzzCorpusTests.cs

@@ -0,0 +1,36 @@
+using PalladiumWallet.Fuzz;
+
+namespace PalladiumWallet.Tests;
+
+/// <summary>
+/// Replays the fuzzing seed corpus (tests/PalladiumWallet.Fuzz/Corpus, copied
+/// next to the test binary) through every fuzz target on each test run: the
+/// corpus includes the regression inputs for crashes found by past campaigns,
+/// so a fixed contract violation can never silently come back. The real
+/// coverage-guided campaigns run separately via tests/PalladiumWallet.Fuzz/fuzz.sh.
+/// </summary>
+public class FuzzCorpusTests
+{
+    public static TheoryData<string> Targets()
+    {
+        var data = new TheoryData<string>();
+        foreach (var name in FuzzTargets.All.Keys)
+            data.Add(name);
+        return data;
+    }
+
+    [Theory]
+    [MemberData(nameof(Targets))]
+    public void Il_corpus_del_target_rispetta_il_contratto_del_parser(string target)
+    {
+        var dir = Path.Combine(AppContext.BaseDirectory, "Corpus", target);
+        Assert.True(Directory.Exists(dir), $"seed corpus missing for '{target}' at {dir}");
+
+        foreach (var file in Directory.EnumerateFiles(dir))
+        {
+            // Any escaping exception is a contract violation: the target itself
+            // swallows the exception types documented as the parser's failure mode.
+            FuzzTargets.Run(target, File.ReadAllBytes(file));
+        }
+    }
+}