datalog: dl-summary inspection helper (173/173)

Returns {<rel-name>: tuple-count} for relations with any tuples
or that are rule-headed (so empty IDB shows as :rel 0 rather than
disappearing). Skips placeholder entries from internal
dl-ensure-rel! calls. 4 tests cover basic, empty IDB, mixed
EDB+IDB, and empty-db cases.

lib/datalog/db.sx

@@ -327,3 +327,40 @@
           (fn (k) (set! total (+ total (len (get facts k)))))
           (keys facts))
         total))))
+
+;; Returns {<rel-name>: tuple-count} for debugging. Includes
+;; relations with any tuples plus all rule-head relations (so empty
+;; IDB shows as 0). Skips empty EDB-only entries that are placeholders
+;; from internal `dl-ensure-rel!` calls.
+(define
+  dl-summary
+  (fn
+    (db)
+    (let
+      ((facts (get db :facts))
+        (out {})
+        (rule-heads (list)))
+      (do
+        (for-each
+          (fn
+            (rule)
+            (let ((h (dl-rel-name (get rule :head))))
+              (when
+                (and (not (nil? h)) (not (dl-member-string? h rule-heads)))
+                (append! rule-heads h))))
+          (dl-rules db))
+        (for-each
+          (fn
+            (k)
+            (let ((c (len (get facts k))))
+              (when
+                (or (> c 0) (dl-member-string? k rule-heads))
+                (dict-set! out k c))))
+          (keys facts))
+        ;; Add rule heads that have no facts (yet).
+        (for-each
+          (fn
+            (k)
+            (when (not (has-key? out k)) (dict-set! out k 0)))
+          rule-heads)
+        out))))