feat(call): implement CallConnection with imported-ops overlay (Layer 2) and call/subscribe/abort methods

Implements CallConnection in src/protocol/connection.rs representing an
established alknet/call connection (either direction). Holds the Layer 2
imported-ops overlay (ADR-024) as Arc<RwLock<HashMap>>.

- register_imported / register_imported_all add to the connection overlay
- overlay_env returns an OperationEnv dispatching to imported ops; contains()
  returns true only for ops in the overlay
- call() opens a stream, sends call.requested, registers in PendingRequestMap,
  spawns a stream reader, resolves on first call.responded
- subscribe() sends call.requested and yields call.responded until
  call.completed/call.aborted via a SubscriptionStream wrapping the mpsc receiver
- abort() sends call.aborted for the request ID and removes the pending entry
- connection drop drops the overlay (no explicit deregistration needed)

Exposes MockConnection trait and Connection::from_mock in alknet-core so
cross-crate tests can construct mock connections without real QUIC. Removes
two unused test helpers in env.rs that triggered dead-code warnings under
-D warnings. Adds parking_lot dep for the overlay RwLock and pending Mutex.

9 new connection tests (102 total in alknet-call). Clippy clean.

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) },