This PR implements the resolution to the following issue, as approved in SysML v2 FTF 2 Ballot 7:

• SYSML2_-262 Flows cannot connect ControlNodes

(This should have been implemented in release 2025-02, but it was missed.)

In the SysML abstract syntax, ControlNodes (i.e., fork, join, decide and merge nodes) are kinds of ActionUsages that are typed by ControlActions from the Systems Model Library. The abstract syntax allows control nodes to have any sort of owned members that other kinds of action usages can. However, the textual notation syntax previously allowed only annotations to be declared in the body of a control node. The resolution to SYSML2_-262 corrects this by revising the SysML textual notation grammar to allow control nodes to have regular action bodies.

In particular, with this change, it is possible to declare control nodes with parameters, enabling the modeling of flows through them. For example:

action def A {
    action a1 {
        out x1;
    }
    then j1;

    action a2 {
        out x2;
    }
    then j1;

    flow a1.x1 to j1.x1;
    flow a2.x2 to j1.x2;

    join j1 {
        in x1;
        in x2;
        out y = (x1, x2);
    }
    then a3;

    flow j1.y to a3.y;

    action a3 {
        in y;
    }
}

Note that it is still necessary to have successions (or succession flows) into and out of control nodes in order to get the proper "control" semantics.