Name the ToString override a node has to carry

[Rafael-SOWNet]

Docs/Contributing/AddingNode.cs exists to list every place a new node must be taught about —
AGENTS.md says to read it before adding one. It does not mention ToString.

Every one of the 69 node types carries public override string ToString() => Stringize();:

$ grep -rn "override string ToString" Sources/AngouriMath --include=*.cs | grep -c "=> Stringize();"
69
$ grep -c "ToString" Sources/AngouriMath/Docs/Contributing/AddingNode.cs
0

Why omitting it is worse than a wrong string

Entity is a record, so C# synthesises ToString from PrintMembers, which prints every public
property. Two of Entity's are Entity-valued:

Functions/Evaluation/Evaluation.Definition.cs:139:  public Entity Evaled => …
Functions/Evaluation/Evaluation.Definition.cs:168:  public Entity InnerSimplified => …

So the synthesised ToString calls ToString on each of them, which calls it on theirs, and so on.
A node missing the override does not print the wrong thing — it exhausts the stack, and it does so
at a call site nowhere near the node, which is the hard kind of failure to trace back.

Nothing enforces it. It is not a compiler error, no analyzer checks it, and the 69 existing
occurrences are the only reason it has never bitten: every node was copied from one that had it,
which is exactly the kind of invariant that survives until the first person who does not copy.

Scope

One line of guidance in AddingNode.cs, +6 lines. No code, no behaviour change, no
BREAKING-CHANGES.md entry. Touches no file any other open PR touches.

Found while measuring what an Entity subclass outside the kernel assembly needs in order to work —
#1026, and the design document for it is
#1040.