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/common-lisp/clos.sx

@@ -330,37 +330,22 @@
                     false))))))
         (check-all 0)))))
 
-;; Precedence distance: how far class-name is from spec-name up the hierarchy.
-(define
-  clos-specificity
-  (let
-    ((registry clos-class-registry))
-    (fn
-      (class-name spec-name)
-      (define
-        walk
-        (fn
-          (cn depth)
-          (if
-            (= cn spec-name)
-            depth
-            (let
-              ((rec (get registry cn)))
-              (if
-                (nil? rec)
-                nil
-                (let
-                  ((results (map (fn (p) (walk p (+ depth 1))) (get rec "parents"))))
-                  (let
-                    ((non-nil (filter (fn (x) (not (nil? x))) results)))
-                    (if
-                      (empty? non-nil)
-                      nil
-                      (reduce
-                        (fn (a b) (if (< a b) a b))
-                        (first non-nil)
-                        (rest non-nil))))))))))
-      (walk class-name 0))))
+;; CLOS-side adapter for lib/guest/reflective/class-chain.sx. Classes
+;; live in clos-class-registry; :parents is a list of parent class
+;; names (CLOS supports multiple inheritance).
+(define clos-class-cfg
+  {:parents-of (fn (cn)
+                 (let ((rec (clos-find-class cn)))
+                   (cond ((nil? rec) (list))
+                         (:else (or (get rec "parents") (list))))))
+   :class?     (fn (n) (not (nil? (clos-find-class n))))})
+
+;; Precedence distance: how far class-name is from spec-name up the
+;; hierarchy. Delegates to refl-class-chain-depth-with which handles
+;; the multi-parent DFS with min-depth selection.
+(define clos-specificity
+  (fn (class-name spec-name)
+    (refl-class-chain-depth-with clos-class-cfg class-name spec-name)))
 
 (define
   clos-method-more-specific?

lib/common-lisp/test.sh

@@ -368,7 +368,7 @@ run_program_suite \
 
 # ── Phase 4: CLOS unit tests ─────────────────────────────────────────────────
 CLOS_FILE=$(mktemp); trap "rm -f $CLOS_FILE" EXIT
-printf '(epoch 1)\n(load "spec/stdlib.sx")\n(epoch 2)\n(load "lib/common-lisp/runtime.sx")\n(epoch 3)\n(load "lib/common-lisp/clos.sx")\n(epoch 4)\n(load "lib/common-lisp/tests/clos.sx")\n(epoch 5)\n(eval "passed")\n(epoch 6)\n(eval "failed")\n(epoch 7)\n(eval "failures")\n' > "$CLOS_FILE"
+printf '(epoch 1)\n(load "spec/stdlib.sx")\n(epoch 2)\n(load "lib/common-lisp/runtime.sx")\n(epoch 3)\n(load "lib/guest/reflective/class-chain.sx")\n(load "lib/common-lisp/clos.sx")\n(epoch 4)\n(load "lib/common-lisp/tests/clos.sx")\n(epoch 5)\n(eval "passed")\n(epoch 6)\n(eval "failed")\n(epoch 7)\n(eval "failures")\n' > "$CLOS_FILE"
 CLOS_OUT=$(timeout 30 "$SX_SERVER" < "$CLOS_FILE" 2>/dev/null)
 rm -f "$CLOS_FILE"
 CLOS_PASSED=$(echo "$CLOS_OUT" | grep -A1 "^(ok-len 5 " | tail -1 || true)
@@ -389,7 +389,7 @@ fi
 run_clos_suite() {
   local prog="$1" pass_var="$2" fail_var="$3" failures_var="$4"
   local PROG_FILE=$(mktemp)
-  printf '(epoch 1)\n(load "spec/stdlib.sx")\n(epoch 2)\n(load "lib/common-lisp/runtime.sx")\n(epoch 3)\n(load "lib/common-lisp/clos.sx")\n(epoch 4)\n(load "%s")\n(epoch 5)\n(eval "%s")\n(epoch 6)\n(eval "%s")\n(epoch 7)\n(eval "%s")\n' \
+  printf '(epoch 1)\n(load "spec/stdlib.sx")\n(epoch 2)\n(load "lib/common-lisp/runtime.sx")\n(epoch 3)\n(load "lib/guest/reflective/class-chain.sx")\n(load "lib/common-lisp/clos.sx")\n(epoch 4)\n(load "%s")\n(epoch 5)\n(eval "%s")\n(epoch 6)\n(eval "%s")\n(epoch 7)\n(eval "%s")\n' \
     "$prog" "$pass_var" "$fail_var" "$failures_var" > "$PROG_FILE"
   local OUT; OUT=$(timeout 20 "$SX_SERVER" < "$PROG_FILE" 2>/dev/null)
   rm -f "$PROG_FILE"