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.