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/tests/semi_naive.sx

@@ -120,18 +120,20 @@
         (dl-sn-counts-agree?
           (dl-sn-counts "p(a). p(b). q(X) :- p(X), =(X, a)."))
         true)
-      ;; Chain length 10 — exercises multiple semi-naive iterations
-      ;; against the recursive ancestor rule.
+      ;; Chain length 12 — multiple semi-naive iterations against
+      ;; the recursive ancestor rule (differential vs naive).
       (dl-sn-test!
-        "chain-10 ancestor counts match"
-        (dl-sn-counts-agree? (dl-sn-counts (dl-sn-chain-source 10)))
+        "chain-12 ancestor counts match"
+        (dl-sn-counts-agree? (dl-sn-counts (dl-sn-chain-source 12)))
         true)
+      ;; Chain length 25 — semi-naive only — first-arg index makes
+      ;; this tractable in conformance budget.
       (dl-sn-test!
-        "chain-15 ancestor count value (semi only)"
+        "chain-25 ancestor count value (semi only)"
         (let
-          ((db (dl-program (dl-sn-chain-source 15))))
+          ((db (dl-program (dl-sn-chain-source 25))))
           (do (dl-saturate! db) (len (dl-relation db "ancestor"))))
-        120)
+        325)
       (dl-sn-test!
         "query through semi saturate"
         (let