Chez Phase 1 (increment 3q): multimethod dispatch + late-bind

host/chez/multimethods.ss implements the multimethod runtime: defmulti/defmethod
expand to defmulti-setup/defmethod-setup calls (+ get-method/methods/
remove-method/prefer-method/prefers). A jolt-multifn record carries its dispatch
fn and a jolt=-keyed method table; jolt-invoke dispatches it (exact match, then
isa?/hierarchy with prefer-method, then :default), reusing the overlay's
isa?/derive/make-hierarchy. The multifn's ns comes from a runtime chez-current-ns
(default user; the prelude load sets clojure.core for print-method/print-dup).

Two emit-side changes were needed:
- late-bind (:late-bind-unresolved? ctx flag, default OFF): defmulti expands to a
  bare-symbol setup call, so the analyzer doesn't intern the name and a forward
  reference '(area ...)' after '(defmulti area ...)' in one form was 'Unable to
  resolve symbol'. The strict compiler punts these to the interpreter; the Chez
  back end has none, so the flag lowers an unresolved symbol to a var-ref against
  the compile ns (open-world -e semantics). Set only by the Chez make-ctx /
  jolt-chez; the main compiler keeps strict resolution (host_iface late-bind?
  defaults nil).
- a :var call head now routes through jolt-invoke, since a late-bound var can hold
  a multifn (or keyword/coll IFn), not just a procedure. Transparent for
  procedures; the hot self-recursive call is a :local known-proc, stays direct.

Class-based dispatch ((class x)/String) deferred (needs deftype/class subsystem).

Parity 1506 -> 1530/2497, 0 new divergences. emit-test 302/302. Full janet gate
green (the analyzer flag is off there; suite flakiness under parallel load only).

test/chez/emit-test.janet

@@ -583,6 +583,32 @@
       (ok (string "y1zq -e: " src) (and (= code 0) (= out want))
           (string "chez=" out " janet=" want " | " err)))))
 
+# 3v) multimethod dispatch (jolt-9ls5): defmulti/defmethod expand to
+#   defmulti-setup/defmethod-setup (+ get-method/methods/remove-method/
+#   prefer-method/prefers); host/chez/multimethods.ss provides the runtime. A
+#   jolt-multifn record carries its dispatch fn + method table; jolt-invoke
+#   dispatches it (direct match, then isa?/hierarchy + prefers, then :default).
+#   Dispatch uses the overlay isa?/derive/make-hierarchy, so these need the full
+#   prelude -> the -e binary. (Class-based dispatch — (class x)/String — is
+#   deferred; it needs the deftype/class subsystem.)
+(when (os/stat "bin/jolt-chez")
+  (each src ["(= \"two\" (do (defmulti f identity) (defmethod f 1 [_] \"one\") (defmethod f 2 [_] \"two\") (f 2)))"
+             "(= \"circle\" (do (defmulti area :shape) (defmethod area :circle [_] \"circle\") (area {:shape :circle})))"
+             "(= \"other\" (do (defmulti f identity) (defmethod f 1 [_] \"one\") (defmethod f :default [_] \"other\") (f 99)))"
+             "(= 5 (do (defmulti g (fn [a b] a)) (defmethod g :add [_ b] b) (g :add 5)))"
+             "(= :is-shape (do (derive :hsq :hshape) (defmulti hmm identity) (defmethod hmm :hshape [_] :is-shape) (hmm :hsq)))"
+             "(= :parent (do (def hh (atom (derive (make-hierarchy) :c :p))) (defmulti cmm identity :hierarchy hh) (defmethod cmm :p [_] :parent) (cmm :c)))"
+             "(= :exact (do (derive :de1 :de2) (defmulti emm identity) (defmethod emm :de2 [_] :parent) (defmethod emm :de1 [_] :exact) (emm :de1)))"
+             "(= \"one\" (do (defmulti f identity) (defmethod f 1 [_] \"one\") ((get-method f 1) 1)))"
+             "(= \"one\" (do (defmulti f identity) (defmethod f 1 [_] \"one\") ((get (methods f) 1) 1)))"
+             "(= 2 (do (defmulti f identity) (defmethod f 1 [_] \"one\") (defmethod f 2 [_] \"two\") (count (methods f))))"]
+    (let [[code out err] (run-jolt-chez src) want (cli-oracle src)]
+      (ok (string "multimethod: " src) (and (= code 0) (= out want))
+          (string "chez=" out " janet=" want " | " err))))
+  # no-match throws (exits non-zero), like the corpus :throws row.
+  (let [[code out err] (run-jolt-chez "(do (defmulti f identity) (defmethod f 1 [_] \"one\") (f 99))")]
+    (ok "multimethod: no match throws" (not= code 0) (string "code=" code))))
+
 # 4) perf signal: emitted fib(30) in-Scheme timing (excludes Chez startup), to
 #    track against the spike ceiling (hand-Scheme fib ~5ms). Informational — the
 #    jolt-truthy? wrapper (~3x) and flonum modeling are known Phase-4 levers.