Eliminate `span(span :val)` problem

[wkhere]

originally I named it: "Eliminate span(span :val) problem by forbidding it during compilation".

Noble aim, but seems close to impossible with the current Core, where actual DOM building happens at runtime.

Renaming to something more close to reality, like: detecting it in Core during Agent updates.

Would anyway require carrying some additional state of the current scope, or some scanning hackery done on the scopes stack.