Cross-ns def/require/use/defmethod through the spine (jolt-c2l1)

The per-form eval passed a FIXED compile-ns to every subform of a top-level do,
so a runtime (ns ...)/(in-ns ...) didn't redirect later defs/refs — defs landed
in the wrong ns and qualified refs hit host-static ("Unknown class"). Thread
the current ns: each subform analyzes in (chez-current-ns), which ns/in-ns move.

That exposed two more gaps, now fixed:
- use refers ALL of a target's public vars (a refer-all table consulted by
  chez-resolve-refer) — was bound to plain require (explicit :refer only).
- defmethod on a QUALIFIED multifn (cf.mm/ext from another ns) resolves in the
  symbol's ns, not the current one (was auto-creating a stray multifn).

Corpus 2684->2688, 0 new divergences; floor raised. No re-mint (runtime shims).

host/chez/natives-str.ss

@@ -286,4 +286,24 @@
   (for-each (lambda (s) (chez-register-spec! (chez-current-ns) s)) specs)
   jolt-nil)
 (def-var! "clojure.core" "require" chez-runtime-require)
-(def-var! "clojure.core" "use" chez-runtime-require)
+;; use = require + refer ALL of the target's public vars (unless an explicit
+;; :only/:refer filter is given, which chez-register-spec! handles per-name).
+(define (chez-runtime-use . specs)
+  (for-each
+    (lambda (spec)
+      (chez-register-spec! (chez-current-ns) spec)
+      (let* ((items (cond ((pvec? spec) (seq->list spec))
+                          ((or (cseq? spec) (empty-list-t? spec)) (seq->list spec))
+                          ((symbol-t? spec) (list spec))
+                          (else '())))
+             (target (and (pair? items) (symbol-t? (car items)) (symbol-t-name (car items))))
+             (filtered (let scan ((xs (if (pair? items) (cdr items) '())))
+                         (cond ((null? xs) #f)
+                               ((and (keyword? (car xs))
+                                     (member (keyword-t-name (car xs)) '("only" "refer"))) #t)
+                               (else (scan (cdr xs)))))))
+        (when (and target (not filtered))
+          (chez-register-refer-all! (chez-current-ns) target))))
+    specs)
+  jolt-nil)
+(def-var! "clojure.core" "use" chez-runtime-use)