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.
