Add the receiver's possible states to the error context #10

The VS Code State Machine view needs the states the receiver can be in at a failing call, so it can split them into the states the call accepts and the states it rejects (liquid-java/vscode-liquidjava#158).

Given:

• P: the states the receiver can be in, from the Found condition;
• A: the states the call accepts, from the Expected condition (expectedStates).

The view shows Valid = P ∩ A and Invalid = P \ A.

actualStates only covers some forms of Found and is not documented as P. A dedicated field states exactly what the view relies on.

Proposal

Add possibleStates to StateMachineErrorContext: the states, in state set order, that the receiver's Found premises allow. Use the same evaluation as expectedStates (||, !, ternaries), and intersect across conjoined premises. When nothing is known about the receiver, return every state.

Tests

• DatagramSocket after if (exit) socket.close(); else socket.send(p); then socket.receive(p). Found is flag ? closed(socket) : bound(socket), so P = {bound, closed} and A = {unbound, bound, connected}. This is the vscode-liquidjava test example E_OSS_A1_22_UdpChatClient.
• A negated Found, such as !closed(s).
• The existing DefaultMutableTreeNode and Door fixtures.