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-core/src/types.rs

@@ -368,7 +368,7 @@ enum ConnectionKind {
 }
 
 #[allow(dead_code)]
-pub(crate) trait MockConnection: Send + Sync {
+pub trait MockConnection: Send + Sync {
     fn remote_alpn(&self) -> &[u8];
     fn remote_addr(&self) -> Option<SocketAddr>;
     fn close(&self, code: u32, reason: &str);
@@ -406,7 +406,7 @@ impl Connection {
     }
 
     #[allow(dead_code)]
-    pub(crate) fn from_mock(mock: Arc<dyn MockConnection + Send + Sync>) -> Self {
+    pub fn from_mock(mock: Arc<dyn MockConnection + Send + Sync>) -> Self {
         let alpn = mock.remote_alpn().to_vec();
         Self {
             kind: ConnectionKind::Mock(mock),
@@ -523,6 +523,7 @@ mod tests {
         closed: std::sync::Mutex<Option<(u32, String)>>,
     }
 
+    #[allow(dead_code)]
     impl MockConnection for MockConn {
         fn remote_alpn(&self) -> &[u8] {
             self.alpn