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/magic.sx

@@ -158,3 +158,45 @@
                       (dl-vars-bound-by-lit lit bound)))))))
           (get rule :body))
         out))))
+
+;; ── Magic predicate naming + bound-args extraction ─────────────
+;; These are building blocks for the magic-sets *transformation*
+;; itself. The transformation (which generates rewritten rules
+;; with magic_<rel>^<adornment> filters) is future work — for now
+;; these helpers can be used to inspect what such a transformation
+;; would produce.
+
+;; "magic_p^bf" given relation "p" and adornment "bf".
+(define
+  dl-magic-rel-name
+  (fn (rel adornment) (str "magic_" rel "^" adornment)))
+
+;; A magic predicate literal:
+;;   (magic_<rel>^<adornment> arg1 arg2 ...)
+(define
+  dl-magic-lit
+  (fn
+    (rel adornment bound-args)
+    (cons (string->symbol (dl-magic-rel-name rel adornment)) bound-args)))
+
+;; Extract bound args (those at "b" positions in `adornment`) from a
+;; literal `(rel arg1 arg2 ... argN)`. Returns the list of arg values.
+(define
+  dl-bound-args
+  (fn
+    (lit adornment)
+    (let ((args (rest lit)) (out (list)))
+      (do
+        (define
+          dl-ba-loop
+          (fn
+            (i)
+            (when
+              (< i (len args))
+              (do
+                (when
+                  (= (slice adornment i (+ i 1)) "b")
+                  (append! out (nth args i)))
+                (dl-ba-loop (+ i 1))))))
+        (dl-ba-loop 0)
+        out))))