New flow: connect -> OpGetChallenge -> OpAuthCert (signature + certDER) -> server verifies cert chain + Ed25519 signature over challenge data -> reuses existing identity. No re-registration on reconnect.
Store challenge from handleGetChallenge in session, then verify
against it in handleSolve. Previously a fresh challenge was
generated for verification, making auth trivially bypassable.
Add 'qcc' ALPN protocol to both ACME and insecure TLS configs.
Bump golang.org/x/crypto and transitive x/ deps.
Add golang.org/x/mobile for mobile client bindings.
