Unify unmatched superclass with context predicate node

Unify unmatched superclass with context predicate node

Was: unmatched superclass → class node, edge "needs Foo a b (no
local match)".
Now: unmatched superclass → predicate node (the same one created
from a context constraint when the predicate happens to match,
otherwise a fresh role='extern' grey node), edge labelled
"superclass constraint".

This collapses the case where a constraint appears in *both* the
context and as an unmatched superclass — the user sees one node
with two edges ("instance context" + "superclass constraint")
instead of two disconnected nodes saying the same thing. When the
constraint is purely an external superclass, the grey 'extern'
predicate node makes "must exist somewhere outside this project"
visible at a glance.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>