reflective: extract class-chain.sx — Smalltalk + CLOS method dispatch share parent-walk

lib/guest/reflective/class-chain.sx — class inheritance walker with
adapter cfg for single-parent (Smalltalk) and multi-parent (CLOS)
hierarchies. Three primitives:

- refl-class-chain-find-with CFG CN PROBE
    DFS through parents, returns first non-nil probe result.
    Smalltalk method lookup uses this.

- refl-class-chain-depth-with CFG CN ANCESTOR
    Min hop distance via any parent path, or nil if unreachable.
    CLOS method specificity uses this.

- refl-class-chain-ancestors-with CFG CN
    Flat DFS-ordered list of all reachable ancestor names.

Adapter cfg has two keys: :parents-of (CN → list of parent names,
possibly empty) and :class? (predicate; short-circuits walk on
non-existent class names mid-chain).

Migrations:
- lib/smalltalk/runtime.sx: st-method-lookup-walk now a 9-line
  thin probe through the kit (was 20 lines of inline recursion);
  st-class-cfg wraps the single-parent :superclass field into a
  1-element list for the cfg.

- lib/common-lisp/clos.sx: clos-specificity is a one-line wrapper
  around refl-class-chain-depth-with (was 28 lines); clos-class-cfg
  reads the multi-parent :parents field.

Both consumers green:
- Smalltalk: 847/847 (unchanged)
- CL: 222/240 (unchanged baseline; 18 pre-existing failures, all
  in stdlib functions like cl-set-memberp, unrelated to CLOS).

This is the second extracted reflective kit (env.sx was first).
The adapter-cfg pattern continues to bridge structurally divergent
consumers (Smalltalk single-inheritance vs CLOS multiple-inheritance
with method-precedence distance) via a uniform :parents-of callback.

lib/smalltalk/runtime.sx

@@ -221,30 +221,37 @@
                     (st-ic-bump-generation!)
                     true))))))))))
 
+;; Smalltalk-side adapter for lib/guest/reflective/class-chain.sx.
+;; Smalltalk has single inheritance: :parents-of returns a 1-element
+;; list (or empty) wrapping the single :superclass field.
+(define st-class-cfg
+  {:parents-of (fn (cn)
+                 (let ((p (st-class-superclass cn)))
+                   (cond ((nil? p) (list))
+                         (:else (list p)))))
+   :class?     (fn (n) (st-class-exists? n))})
+
 ;; Walk-only lookup. Returns the method record (with :defining-class) or nil.
 ;; class-side? = true searches :class-methods, false searches :methods.
+;; Parent-chain walk delegates to refl-class-chain-find-with; the probe
+;; tests this class's method dict and returns the entry (or nil).
 (define
   st-method-lookup-walk
-  (fn
-    (cls-name selector class-side?)
-    (let
-      ((found nil))
-      (begin
-        (define
-          ml-loop
-          (fn
-            (cur)
-            (when
-              (and (= found nil) (not (= cur nil)) (st-class-exists? cur))
-              (let
-                ((c (st-class-get cur)))
-                (let
-                  ((dict (if class-side? (get c :class-methods) (get c :methods))))
-                  (cond
-                    ((has-key? dict selector) (set! found (get dict selector)))
-                    (else (ml-loop (get c :superclass)))))))))
-        (ml-loop cls-name)
-        found))))
+  (fn (cls-name selector class-side?)
+    (refl-class-chain-find-with
+      st-class-cfg
+      cls-name
+      (fn (cn)
+        (let ((c (st-class-get cn)))
+          (cond
+            ((nil? c) nil)
+            (:else
+              (let ((dict (if class-side?
+                            (get c :class-methods)
+                            (get c :methods))))
+                (cond
+                  ((has-key? dict selector) (get dict selector))
+                  (:else nil))))))))))
 
 ;; Cached lookup. Misses are stored as :not-found so doesNotUnderstand paths
 ;; don't re-walk on every send.