datalog: dl-magic-query driver (204/204)

End-to-end magic-sets entry point. Given (db, query-goal):
  - copies the caller's EDB facts (relations not headed by any
    rule) into a fresh internal db
  - adds the magic seed fact
  - adds the rewritten rules
  - saturates and runs the query
  - returns the substitution list

Caller's db is untouched. Equivalent to dl-query for any
fully-stratifiable program; intended as a perf alternative on
goal-shaped queries against large recursive relations.

2 new tests: equivalence to dl-query on chain-3 ancestor, and
non-mutation of the caller's db (rules count unchanged).

plans/datalog-on-sx.md

@@ -174,11 +174,15 @@ large graphs.
 - [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).
+- [x] `dl-magic-query db query-goal` — top-level driver. Builds a
+      fresh internal db with the caller's EDB facts, the magic seed,
+      and the rewritten rules; saturates and queries. Caller's db is
+      untouched. Equivalent to `dl-query` for fully-stratifiable
+      programs (sole motivation is a perf alternative on goal-shaped
+      queries against large recursive relations).
 - [ ] 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.
+      Left to a future iteration — would need a benchmarking harness
+      for large graphs and the conformance budget can't afford it.
 
 ### Phase 7 — stratified negation
 - [x] Dependency graph: `dl-build-dep-graph db` returns `{head -> ({:rel
@@ -299,6 +303,14 @@ large graphs.
 
 _Newest first._
 
+- 2026-05-08 — Phase 6 driver: `dl-magic-query db query-goal`.
+  Builds a fresh internal db from the caller's EDB + magic seed +
+  rewritten rules, saturates, queries, returns substitutions —
+  caller's db is untouched. Equivalent to `dl-query` for any
+  fully-stratifiable program; sole motivation is a perf alternative
+  on goal-shaped queries against large recursive relations.
+  2 new tests cover equivalence and non-mutation.
+
 - 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