Phase B1: connectivity checking and signed peer descriptors

[ptesavol]

Implements Phase B1 of trackerless-network-completion-plan.md (Milestone B — direct connectivity): the NAT connectivity-check protocol, signed peer descriptors, and the websocket reverse-connection path, ported line-by-line from the pinned TypeScript (af966cf03, v103.8.0-rc.3).

Part 1 — new modules

• connectivityChecker (client side): connectSync (TS connectAsync) opens a raw websocket and waits for it to come up; sendConnectivityRequest runs the check against one entry point and returns its ConnectivityResponse. Deliberately synchronous-blocking: both entry points run on dedicated worker threads, never on an event-dispatch or RPC thread; listeners are registered before connect()/send() so a fast completion cannot be lost.
• connectivityRequestHandler (server side): attached to incoming connections carrying the connectivityRequest action; probes the requester's advertised websocket server (connectivityProbe action) unless probing is disabled (port 0), and replies with a ConnectivityResponse. Runs on a dedicated worker executor so the probe cannot stall the websocket dispatch thread. GeoIP is omitted (deferred to milestone E).
• createPeerDescriptor + createPeerDescriptorSignaturePayload: the signed local peer descriptor. Node id = concat(last 13 bytes of keccak(ip), last 7 bytes of sign(ip)); the keccak and signature use the Ethereum message magic (TS EcdsaSecp256k1Evm defaults) via SigningUtils — wire-exact. Like TS (its TODO), the key pair is throwaway.
• New error types ConnectionFailed, ConnectivityResponseTimeout.

Part 2 — wiring and reverse connections

• WebsocketServerConnector.start(): the connectivityRequest action now attaches the handler; connectivityProbe sockets are pinned by a listener until the prober closes them (C++ has no GC to keep an unowned server socket alive; onClosed drops all listeners, releasing the pin).
• WebsocketServerConnector.checkConnectivity(): real flow — state snapshot under the lock, network outside it; shuffled entry points; 2 s abortable wait between failed attempts; WebsocketServerStartError with the attempted hosts after the list is exhausted. The no-entry-points branch tolerates a serverless connector (empty host/port, like the TS undefined fields).
• Reverse connections (connect / isPossibleToFormConnection / requestConnectionFromPeer): a node with a websocket server asks a serverless peer over the signaling transport to open a websocket back to it; the pending connection completes when the incoming handshake matches ongoingConnectRequests. The notification runs detached on a dedicated executor that destroy() joins after abort (the PeerManager::stop() pattern).
• DefaultConnectorFacade: routes createConnection through the server connector when the client connector cannot form the connection (TS falls through to WebRTC next — milestone B2), and passes the new entryPoints option through. externalIp only feeds the WebRTC connector in TS, so it lands with B2; TLS/auto-certify options stay deferred.

Two real bugs found by the new tests

1. WebsocketClientConnectorRpcRemote::requestConnection SEGV on first real use. It returned the generated client's task, but RpcCommunicatorClientApi::notify is a lazy coroutine taking const RequestType& — by the time the task first ran, the locals it referenced were destroyed (ASan: SEGV in Any::PackFrom). Now a proper coroutine: co_await keeps the locals alive in its own frame. Repo swept for other return this->getClient()... instances — this was the only one.
2. Connectivity-request server sockets had no owner. attachConnectivityRequestHandler captured the connection weakly, so the socket was destroyed as soon as WebsocketServer released its half-ready reference — the request never got processed (the handshake path survives because the handshaker owns its socket; TS survives on GC). The Data listener now holds the connection strongly; the cycle is broken by onClosed's remove-all-listeners. The executor task still captures weakly, so a connection that closes while queued is simply dropped.

Tests

Ported (v103.8.0-rc.3): unit/createPeerDescriptor.test.ts, unit/connectivityRequestHandler.test.ts (real websocket server as probe target), integration/ConnectivityChecking.test.ts, integration/Websocket.test.ts (round-trip case added to the existing WebsocketClientServerTest), integration/WebsocketConnectionManagement.test.ts (all five cases, including both serverless reverse-connection directions and connect-to-self rejection). unit/ConnectivityHelpers.test.ts was already covered by the existing ConnectivityTest.

• Unit suite: 177/177; integration suite: 31/31; the new tests also pass under AddressSanitizer.
• lint.sh green; the three new test files that trip the known clangd std-type-unification false positive are added to the documented exclusion list (compiler builds and runs them; clang-format still checks them).

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