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

Cargo.lock

@@ -55,6 +55,7 @@ dependencies = [
  "async-trait",
  "futures",
  "irpc",
+ "parking_lot",
  "serde",
  "serde_json",
  "thiserror 2.0.18",