Implement CallConnection in protocol/connection.rs with Layer 2 imported-ops
overlay (Arc<RwLock<HashMap>>), register_imported/register_imported_all,
overlay_env() returning an OperationEnv that dispatches to imported ops,
and call()/subscribe()/abort() methods that open a stream, send call.requested,
register in PendingRequestMap, spawn a stream reader, and correlate responses
by ID. Connection drop drops the overlay. Exposed MockConnection +
Connection::from_mock in alknet-core for cross-crate testing. 9 new connection
tests (102 total in alknet-call).
Refs: docs/architecture/crates/call/call-protocol.md
Implements: ADR-012, ADR-017, ADR-024