Show tyvars on class node labels

Show tyvars on class node labels

Class nodes now read "BftCrypto c" / "Bar a b" instead of just the
class name, in both the classes view (Render.hs's classNodes) and
the instance-view ensureClassNode helpers. External classes — for
which we don't know the tyvars — still fall back to the bare name.

Edge labels (the positional substitution into a superclass's args)
keep using the *origin* class's tyvar names via edgeLabel, so an
arrow `Bar a b ──(b)──► Foo b` reads consistently — same letter on
edge and target node.

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