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.

plans/datalog-on-sx.md

@@ -154,10 +154,13 @@ large graphs.
 - [x] Adornments: `dl-adorn-goal goal` and `dl-adorn-lit lit bound` in
       `lib/datalog/magic.sx`. Per-arg `b`/`f` based on whether the arg
       is a constant or a variable already in the bound set.
-- [ ] Magic transformation: for each adorned predicate, generate a
-      `magic_<pred>` relation and rewrite rule bodies to filter through it.
-      *Building blocks present in `magic.sx`: `dl-magic-rel-name`,
-      `dl-magic-lit`, `dl-bound-args`. Full rewriter still TODO.*
+- [x] Magic transformation: `dl-magic-rewrite rules query-rel adn args`
+      generates `{:rules <rewritten-rules> :seed <magic-seed>}`. Each
+      original rule is gated with a `magic_<rel>^<adn>(bound)` filter,
+      and propagation rules are emitted for each positive non-builtin
+      body literal. Worklist over `(rel, adn)` pairs starts from the
+      query and stops when no new pairs appear. EDB facts pass through
+      unchanged.
 - [x] Sideways information passing strategy (SIPS): left-to-right
       `dl-rule-sips rule head-adornment` walks body literals tracking
       the bound set, returning `({:lit :adornment} ...)`. Recognises
@@ -168,9 +171,14 @@ large graphs.
       `:semi-naive`. `:magic` accepted but the transformation itself is
       deferred — saturator currently falls back to semi-naive. Tests
       verify hook, default, and equivalence under the alternate setting.
-- [ ] Tests: equivalence vs naive on small inputs; perf win on a 10k-node
-      reachability query from a single root. *Pending real magic-set
-      transformation.*
+- [x] Equivalence test: rewritten ancestor program over the same EDB
+      derives the same number of `ancestor` tuples and returns the
+      same query answers as the unrewritten program (chain-3 case).
+- [ ] Perf test: 10k-node reachability with magic vs semi-naive.
+      Pending — would need a `dl-saturate-magic!` driver that builds
+      a temporary db from rewrite output. The rewriter itself is in
+      place; benchmarking against semi-naive on large graphs is left
+      to a future iteration.
 
 ### Phase 7 — stratified negation
 - [x] Dependency graph: `dl-build-dep-graph db` returns `{head -> ({:rel
@@ -291,6 +299,17 @@ large graphs.
 
 _Newest first._
 
+- 2026-05-08 — Phase 6 magic-sets rewriter. `dl-magic-rewrite rules
+  query-rel adn args` returns `{:rules <rewritten> :seed <seed-fact>}`.
+  Worklist over `(rel, adn)` pairs starts from the query, gates each
+  original rule with a `magic_<rel>^<adn>(bound)` filter, and emits
+  propagation rules for each positive non-builtin body literal so
+  that magic spreads to body relations. EDB facts pass through.
+  3 new tests cover seed structure, equivalence on chain-3 by
+  ancestor-relation tuple count, and same-query-answers under
+  the rewritten program. The plumbing for a `dl-saturate-magic!`
+  driver and large-graph perf benchmarks is still future work.
+
 - 2026-05-08 — Phase 6 building blocks for the magic-sets
   transformation: `dl-magic-rel-name`, `dl-magic-lit`,
   `dl-bound-args`. The rewriter that generates magic seed and