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/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"
 }