datalog: indexed dl-find-bindings + chain-15 differential (Phase 5c, 153/153)

dl-find-bindings now uses dl-fb-aux lits db subst i n (indexed
iteration via nth) instead of recursive (rest lits). Eliminates
O(N²) list-copy per body of length N. chain-15 saturation 25s
→ 16s; chain-25 finishes in 33s real (vs. timeout previously).

Bumped semi_naive tests to chain-10 differential + chain-15
semi-only count (was chain-5/chain-5). Blocker entry refreshed.

lib/datalog/eval.sx

@@ -83,14 +83,18 @@
 
 (define
   dl-find-bindings
+  (fn (lits db subst) (dl-fb-aux lits db subst 0 (len lits))))
+
+(define
+  dl-fb-aux
   (fn
-    (lits db subst)
+    (lits db subst i n)
     (cond
       ((nil? subst) (list))
-      ((= (len lits) 0) (list subst))
+      ((>= i n) (list subst))
       (else
         (let
-          ((options (dl-match-lit (first lits) db subst))
+          ((options (dl-match-lit (nth lits i) db subst))
             (results (list)))
           (do
             (for-each
@@ -98,7 +102,7 @@
                 (s)
                 (for-each
                   (fn (s2) (append! results s2))
-                  (dl-find-bindings (rest lits) db s)))
+                  (dl-fb-aux lits db s (+ i 1) n)))
               options)
             results))))))