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.

CLAUDE.md

@@ -53,6 +53,7 @@ docker/           reproducible release builds (build.sh + pinned Dockerfiles)
   - Test tree mirrors `src/Core` (`Chain/ Crypto/ Net/ Spv/ Storage/ Wallet/`) — put new tests in the folder matching the code under test
   - Network/SPV code (`ElectrumClient`, `WalletSynchronizer`, `TransactionInspector`, TOFU pinning): test against the in-process fake server `tests/PalladiumWallet.Tests/Net/FakeElectrumServer.cs` (loopback TCP + optional TLS, per-method handlers, call counters) — extend that, don't mock the client (it isn't an interface, by design)
   - Two `internal` test seams (via `InternalsVisibleTo`) sandbox the remaining externals: `UpdateChecker.CheckAsync` takes an optional `HttpMessageHandler` (never hit GitHub from a test); `AppPaths.BootstrapDirOverride`/`PortableBaseOverride`/`DefaultRootOverride` redirect the machine-global path locations (tests using them share xUnit collection `"AppPaths"` because that state is static)
+- **Live server probe** (`tests/PalladiumWallet.ServerProbe`): read-only conformance/health check for real Electrum/Fulcrum/ElectrumX servers (`dotnet run --project tests/PalladiumWallet.ServerProbe -- --server host:port --ssl`, or `--registry`). Also runnable from `dotnet test` via env vars: `PALLADIUM_TEST_SERVERS=host:port,… [PALLADIUM_TEST_SSL=1] [PALLADIUM_TEST_NET=…] dotnet test --filter LiveServer` (no-op when unset, so the default run stays offline)
 - **Fuzzing** (`tests/PalladiumWallet.Fuzz`, SharpFuzz): one target per untrusted-input parser (`header merkle slip132 bip39 address coinamount walletdoc encfile peers`), each encoding that parser's error contract — any other escaping exception is a finding
   - Seed corpus (incl. regression inputs for past findings) replays inside `dotnet test` via `FuzzCorpusTests`
   - Quick smoke without tooling: `dotnet run --project tests/PalladiumWallet.Fuzz -- <target> --random 50000`