datalog: magic-sets rewriter (Phase 6, 202/202)

dl-magic-rewrite rules query-rel adn args returns:
  {:rules <rewritten-rules> :seed <magic-seed-fact>}

Worklist over (rel, adn) pairs starts from the query and stops
when no new pairs appear. For each rule with head matching a
worklist pair:
  - Adorned rule: head :- magic_<rel>^<adn>(bound), body...
  - Propagation rules: for each positive non-builtin body lit
    at position i:
      magic_<lit-rel>^<lit-adn>(bound-of-lit) :-
        magic_<rel>^<adn>(bound-of-head),
        body[0..i-1]
  - Add (lit-rel, lit-adn) to the worklist.

Built-ins, negation, and aggregates pass through without
generating propagation rules. EDB facts are unchanged.

3 new tests cover seed structure, equivalence on chain-3 (full
closure, 6 ancestor tuples — magic helps only when the EDB has
nodes outside the seed's transitive cone), and same-query-answers
under the rewritten program. Total 202/202.

Wiring up a `dl-saturate-magic!` driver and large-graph perf
benchmarks is left for a future iteration.

lib/datalog/scoreboard.md

@@ -1,6 +1,6 @@
 # datalog scoreboard
 
-**199 / 199 passing** (0 failure(s)).
+**202 / 202 passing** (0 failure(s)).
 
 | Suite | Passed | Total | Status |
 |-------|--------|-------|--------|
@@ -13,5 +13,5 @@
 | negation | 10 | 10 | ok |
 | aggregates | 18 | 18 | ok |
 | api | 14 | 14 | ok |
-| magic | 15 | 15 | ok |
+| magic | 18 | 18 | ok |
 | demo | 18 | 18 | ok |