datalog: magic-sets building blocks (199/199)

Adds the primitives a future magic-sets rewriter will compose:

  dl-magic-rel-name rel adornment    → "magic_<rel>^<adornment>"
  dl-magic-lit rel adn bound-args    → magic literal as SX list
  dl-bound-args lit adornment        → bound-position arg values

Rewriter algorithm (worklist over (rel, adornment) pairs,
generating seed, propagation, and adorned-rule outputs) is still
TODO — these helpers are inspection-only for now.

4 new magic tests cover naming, lit construction, and bound-args
extraction (mixed/free).

lib/datalog/tests/magic.sx

@@ -124,7 +124,30 @@
           {:lit (list (quote p) (quote X)) :adornment "f"}
           {:lit (list (quote is) (quote Y)
                       (list (string->symbol "+") (quote X) 1))
-           :adornment "fb"})))))
+           :adornment "fb"}))
+
+      ;; Magic predicate naming.
+      (dl-mt-test! "magic-rel-name"
+        (dl-magic-rel-name "ancestor" "bf")
+        "magic_ancestor^bf")
+
+      ;; Bound-args extraction.
+      (dl-mt-test! "bound-args bf"
+        (dl-bound-args (list (quote ancestor) (quote tom) (quote X)) "bf")
+        (list (quote tom)))
+
+      (dl-mt-test! "bound-args mixed"
+        (dl-bound-args (list (quote p) 1 (quote Y) 3) "bfb")
+        (list 1 3))
+
+      (dl-mt-test! "bound-args all-free"
+        (dl-bound-args (list (quote p) (quote X) (quote Y)) "ff")
+        (list))
+
+      ;; Magic literal construction.
+      (dl-mt-test! "magic-lit"
+        (dl-magic-lit "ancestor" "bf" (list (quote tom)))
+        (list (string->symbol "magic_ancestor^bf") (quote tom))))))
 
 (define
   dl-magic-tests-run!