Don't let a bad multicast interface abort (or terminate) stream resolution

[cboulay]

Summary

Makes stream resolution survive an unusable multicast interface instead of silently abandoning the resolve wave — or crashing the process. This is the best code-level match for "plugging in a VPN / starting Docker breaks LSL discovery."

Root cause

For each multicast send, the resolver selects the outbound interface via multicast_socket_.set_option(outbound_interface(...)) using the throwing overload (src/resolve_attempt_udp.cpp). An interface address that can't be used for IP_MULTICAST_IF — a VPN utun, AWDL, a Hyper-V/VirtualBox adapter, or an address that changed since enumeration — makes this throw. Because it runs inside an asio completion handler:

• in resolve_oneshot, the exception propagates out of io_->run(), aborting the wave and discarding results already received;
• in the continuous resolver, the background thread runs io->run() with no try/catch, so the uncaught exception calls std::terminate() and takes the process down.

(The outlet's IO threads already have a catch-and-retry loop; the resolver's thread was just never given one.)

Fix

• resolve_attempt_udp: use the error_code overload of set_option and, on failure, log at verbosity 1 and carry on. Multicast sends on that pass fall back to the socket's default interface (and individually no-op on send error), while the unicast and broadcast targets — which don't depend on the outbound interface — still go out.
• resolver_impl: wrap the continuous resolver's background io->run() in the same catch-and-retry loop the outlet's IO threads use, so any other stray handler exception can't terminate the process either.

Test

New lsl_test_resolve_bad_interface (own executable, since it sets the api_config singleton): configures an unusable multicast interface (203.0.113.1, TEST-NET-3 per RFC 5737) and asserts a local stream still resolves without crashing. Pre-fix it fails with "An internal error has occurred" (the asio exception thrown out of resolve_oneshot); post-fix it passes.

Testing

Full suite green locally (macOS universal): new test passes; discovery, network, tcpserver, and runtime_config are unaffected.

Scope / relationship

• No wire protocol, API, or config changes.
• Independent of Send resolve queries from the receive socket (fix firewall-asymmetry resolution failures) #278/Add lab.ResolveOverTCP: a firewall-robust TCP-probe discovery fallback #279 and of the sibling PR fixing the same-machine unicast lottery (Resolve all same-machine streams (fix the 127.0.0.1 unicast lottery) #282); the changes don't overlap.