feat(#94): add getClientVersion, getServerProtocol and network thread completion hook

[s2x]

Closes #94

Changes

• FoundationDB::getClientVersion(): string — version/build of the loaded libfdb_c (fdb_get_client_version)
• Database::getServerProtocol(): int — cluster protocol version (fdb_database_get_server_protocol), resolved through the new FutureUInt64 future type binding fdb_future_get_uint64
• FoundationDB::onNetworkThreadCompletion(callable): void — completion hook invoked once from NativeClient::stopNetwork() after the network thread has been joined. The native fdb_add_network_thread_completion_hook() API is deliberately not used for PHP callables: native hooks run on the FDB network thread, where PHP execution is unsafe. The deferred invocation preserves the ordering guarantee (network thread already finished).

Tests

• Unit: FutureUInt64Test (stub-library FFI exercise of fdb_future_get_uint64), NativeClientCompletionHookTest (hooks run once, FIFO, consumed, not run before stop)
• Integration: DatabaseMonitoringTest (getClientVersion non-empty, getServerProtocol > 0), NetworkLifecycleTest (hook runs exactly once on stop, in a separate process)

Docs

• docs/advanced.md — Database Monitoring section extended

Changelog

• [#94] entry added under Unreleased → Added

CI: composer lint and composer test:unit clean locally; full integration suite (242 tests) green against the 5-node Docker cluster.