feat(call): implement CallConnection with imported-ops overlay and call/subscribe/abort (task: call/protocol/call-connection)

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

crates/alknet-call/src/registry/env.rs

@@ -199,30 +199,6 @@ mod tests {
         }
     }
 
-    fn external_spec(name: &str, acl: AccessControl) -> OperationSpec {
-        OperationSpec::new(
-            name,
-            OperationType::Query,
-            Visibility::External,
-            serde_json::json!({}),
-            serde_json::json!({}),
-            vec![],
-            acl,
-        )
-    }
-
-    fn internal_spec(name: &str, acl: AccessControl) -> OperationSpec {
-        OperationSpec::new(
-            name,
-            OperationType::Query,
-            Visibility::Internal,
-            serde_json::json!({}),
-            serde_json::json!({}),
-            vec![],
-            acl,
-        )
-    }
-
     fn echo_handler() -> crate::registry::registration::Handler {
         make_handler(
             |input, context| async move { ResponseEnvelope::ok(context.request_id, input) },