make-hierarchy/derive/underive/isa?/parents/ancestors/descendants are now
Clojure in the overlay — Clojure's own pure-map implementation: a hierarchy
is {:parents {tag #{..}} :ancestors {..} :descendants {..}}, the 3-arity
forms are PURE (derive returns a new hierarchy), and the 1/2-arity forms swap
a private global-hierarchy atom.
This fixes three correctness gaps the Janet kernel had: multi-parent derive
(the kernel's :parents held a single parent per tag), TRANSITIVE descendants
(the kernel tracked direct children only), and vector-pair isa?
((isa? [child1 child2] [parent1 parent2])). Cyclic and duplicate derives now
behave like Clojure (throw / no-op).
Multimethod dispatch was the kernel's only internal caller: defmulti-setup's
dispatch closure now calls the overlay's isa? through a lazily-resolved var
(cached per multimethod); a :hierarchy option is an atom (deref per dispatch,
matching Clojure's var semantics) or a plain hierarchy map. The Janet kernel
(types.janet) and the core-* wrappers are deleted.
20 new hierarchy spec rows (pure 3-arity incl. cycle/duplicate edges, global
+ dispatch incl. custom :hierarchy atoms). Gate green: conformance 326x3,
suite 4566 >= 4540, all batteries, bench in the session band.
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