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.
Host-value protocol dispatch (a protocol extended to Number/String/… called on a
host value) recomputed the candidate type-tag list and walked the registry — up
to ~15 map lookups plus an array alloc — on every call. It now caches
(most-specific-host-tag, protocol, method) -> fn, guarded by a registry
generation that bumps in register-protocol-method; re-extending a protocol
invalidates the cache. The single-lookup fast paths (deftype protocol dispatch,
direct multimethod dispatch) are untouched — they're already cheap, so no cache
overhead there. The multimethod hierarchy-fallback cache (narrower: only
derive-based dispatch) is deferred.
Test in test/integration/dispatch-cache-test.janet covers correctness under
re-extension in all three modes. Conformance 218/218; battery 3916.
