datalog: first-arg index per relation (Phase 5e perf, 169/169)

db gains :facts-index {<rel>: {<first-arg-key>: tuples}} mirroring
the membership :facts-keys index. dl-add-fact! populates the index;
dl-match-positive walks the body literal's first arg under the
current subst — when it's bound to a non-var, look up by (str arg)
instead of scanning the full relation.

For chain-style recursive rules (parent X Y), (ancestor Y Z) the
inner Y has at most one parent, so the inner lookup returns 0–1
tuples instead of N. chain-25 saturation drops from ~33s to ~18s
real (~2x). chain-50 still long but tractable; next bottleneck is
subst dict copies during unification.

dl-retract! refreshed to keep the new index consistent: kept-index
rebuilt during EDB filter, IDB wipes clear all three slots.

Differential semi-naive test bumped to chain-12, semi-only count
test to chain-25.

lib/datalog/eval.sx

@@ -24,7 +24,17 @@
         ((nil? rel) (error (str "dl-match-positive: bad literal " lit)))
         (else
           (let
-            ((tuples (dl-rel-tuples db rel)))
+            ;; If the first argument walks to a non-variable (constant
+            ;; or already-bound var), use the first-arg index for
+            ;; this relation. Otherwise scan the full tuple list.
+            ((tuples
+               (cond
+                 ((>= (len lit) 2)
+                   (let ((walked (dl-walk (nth lit 1) subst)))
+                     (cond
+                       ((dl-var? walked) (dl-rel-tuples db rel))
+                       (else (dl-index-lookup db rel walked)))))
+                 (else (dl-rel-tuples db rel)))))
             (do
               (for-each
                 (fn