datalog: hash-set membership for facts (Phase 5b perf)

db gains a parallel :facts-keys {<rel>: {<tuple-string>: true}}
index alongside :facts. dl-tuple-key derives a stable string via
(str lit) — (p 30) and (p 30.0) collide correctly because SX
prints them identically. dl-add-fact! membership is now O(1)
instead of O(n) list scan; insert sequences for relations sized
N drop from O(N²) to O(N).

Wall clock on chain-7 saturation halves (~12s → ~6s); chain-15
roughly halves (~50s → ~25s) under shared CPU. Larger chains
still slow due to body-join overhead in dl-find-bindings —
Blocker entry refreshed with proposed follow-ups.

dl-retract! keeps both indices consistent: kept-keys is rebuilt
during the EDB filter, IDB wipes clear both lists and key dicts.

lib/datalog/api.sx

@@ -111,21 +111,29 @@
           (has-key? (get db :facts) rel-key)
           (let
             ((existing (get (get db :facts) rel-key))
-              (kept (list)))
+              (kept (list))
+              (kept-keys {}))
             (do
               (for-each
                 (fn
                   (t)
                   (when
                     (not (dl-tuple-equal? t lit))
-                    (append! kept t)))
+                    (do
+                      (append! kept t)
+                      (dict-set! kept-keys (dl-tuple-key t) true))))
                 existing)
-              (dict-set! (get db :facts) rel-key kept))))
+              (dict-set! (get db :facts) rel-key kept)
+              (dict-set! (get db :facts-keys) rel-key kept-keys))))
         ;; Wipe all relations that have a rule (these are IDB) so the
         ;; saturator regenerates them from the surviving EDB.
         (let ((rule-heads (dl-rule-head-rels db)))
           (for-each
-            (fn (k) (dict-set! (get db :facts) k (list)))
+            (fn
+              (k)
+              (do
+                (dict-set! (get db :facts) k (list))
+                (dict-set! (get db :facts-keys) k {})))
             rule-heads))
         (dl-saturate! db)
         db))))