feat(core): implement OperationEnv local dispatch, EventEnvelope, and frame encoding

Add local dispatch for OperationEnv with invoke() method, EventEnvelope
wire format struct, 4-byte BE length-prefixed frame encoding/decoding,
PendingRequestMap for call/subscribe correlation, call protocol event type
constants, and default /services/list and /services/schema operations.

crates/alknet-core/src/call/events.rs

@@ -0,0 +1,28 @@
+pub const CALL_REQUESTED: &str = "call.requested";
+pub const CALL_RESPONDED: &str = "call.responded";
+pub const CALL_COMPLETED: &str = "call.completed";
+pub const CALL_ABORTED: &str = "call.aborted";
+pub const CALL_ERROR: &str = "call.error";
+
+pub const SERVICE_LIST: &str = "/services/list";
+pub const SERVICE_SCHEMA: &str = "/services/schema";
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn event_type_constants() {
+        assert_eq!(CALL_REQUESTED, "call.requested");
+        assert_eq!(CALL_RESPONDED, "call.responded");
+        assert_eq!(CALL_COMPLETED, "call.completed");
+        assert_eq!(CALL_ABORTED, "call.aborted");
+        assert_eq!(CALL_ERROR, "call.error");
+    }
+
+    #[test]
+    fn service_operation_constants() {
+        assert_eq!(SERVICE_LIST, "/services/list");
+        assert_eq!(SERVICE_SCHEMA, "/services/schema");
+    }
+}