D5: directed determination, weak isomorphism, quotient + analytics (prerequisite for D4b)

[cameronfreer]

Sub-issue of the directed umbrella (#84). This is the prerequisite for D4b's uniqueness (see the revised #88): the raw-digraphon mixture representation is non-unique before quotienting.

• directed t, t_inj, t_ind (model: the undirected public triangle, Public without-replacement density triangle: t, t_inj, t_ind (DJ §2) #94);
• componentwise/vector cut norm — the loop coordinate needs separate one-variable control; it must not be hidden in the pair-kernel cut norm;
• directed weak isomorphism and determination;
• DigraphonSpace: the compact quotient and its topology;
• the directed analogue of DJ Theorem 7.1.