The expensive multimethod path is the hierarchy fallback: when a dispatch value
isn't a direct method key, mm-fn walks every key with isa? (derive-based
dispatch). That resolution is now cached per dispatch value on the multimethod
var (:jolt/dispatch-cache), cleared in place by defmethod/prefer-method/
remove-method/remove-all-methods so a redefinition can't be served stale. Direct
(get methods dv) hits stay uncached — already a single lookup.
Test cases added to dispatch-cache-test (compile, interpret, aot-core off):
hierarchy hit is cached, a newly-added specific method is seen, removal re-exposes
the fallback. Conformance 218/218; full suite green.
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