Slice 3 of #94. From the compiled schema, compute exactly what is allowed at an
element, for autocomplete and "what's missing".
- ContentNFA.follow(after:) returns the precise follow-set: the element-name
labels the content automaton can accept next after the children already
present, plus whether the content may legally end there.
- Schema.Completions reports the allowed next elements, the complete flag (false
means a required child is still expected), a wildcard flag, and each declared
attribute with its required and present status.
- CompletionEngine resolves an element's declared type along a coding path (the
global declaration, then each parent's content model) and computes the
completions; Schema.Document.completions(at:in:) is the entry over a ranged
tree.
Tests pin the follow-set as the children accumulate (name then qty then the
optional note, complete flipping true), the attribute required/present status,
and resolution for a nested element by its coding path.
