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))))))
 

lib/datalog/scoreboard.json

@@ -15,5 +15,5 @@
     {"name":"api","passed":9,"failed":0,"total":9},
     {"name":"demo","passed":10,"failed":0,"total":10}
   ],
-  "generated": "2026-05-08T08:45:37+00:00"
+  "generated": "2026-05-08T08:49:52+00:00"
 }

lib/datalog/tests/semi_naive.sx

@@ -120,18 +120,18 @@
         (dl-sn-counts-agree?
           (dl-sn-counts "p(a). p(b). q(X) :- p(X), =(X, a)."))
         true)
-      ;; Chain length 5 — small but enough to exercise multiple
-      ;; semi-naive iterations against a recursive rule.
+      ;; Chain length 10 — exercises multiple semi-naive iterations
+      ;; against the recursive ancestor rule.
       (dl-sn-test!
-        "chain-5 ancestor counts match"
-        (dl-sn-counts-agree? (dl-sn-counts (dl-sn-chain-source 5)))
+        "chain-10 ancestor counts match"
+        (dl-sn-counts-agree? (dl-sn-counts (dl-sn-chain-source 10)))
         true)
       (dl-sn-test!
-        "chain-5 ancestor count value"
+        "chain-15 ancestor count value (semi only)"
         (let
-          ((db (dl-program (dl-sn-chain-source 5))))
+          ((db (dl-program (dl-sn-chain-source 15))))
           (do (dl-saturate! db) (len (dl-relation db "ancestor"))))
-        15)
+        120)
       (dl-sn-test!
         "query through semi saturate"
         (let