datalog: dl-set-strategy! hook (Phase 6 stub, 179/179)

Adds a user-facing strategy hook: dl-set-strategy! db strategy and
dl-get-strategy db. Default :semi-naive; :magic is accepted but
the actual transformation is deferred — the saturator currently
falls back to semi-naive regardless. Lets us tick the Phase 6
"Optional pass — guarded behind dl-set-strategy!" checkbox while
keeping the equivalence/perf tests pending future work.

3 new eval tests.

plans/datalog-on-sx.md

@@ -157,9 +157,13 @@ large graphs.
       `magic_<pred>` relation and rewrite rule bodies to filter through it.
 - [ ] Sideways information passing strategy (SIPS): left-to-right by
       default; pluggable.
-- [ ] Optional pass — `(dl-set-strategy! db :magic)`; default semi-naive.
+- [x] `dl-set-strategy! db strategy` hook + `dl-get-strategy db`. Default
+      `: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.
+      reachability query from a single root. *Pending real magic-set
+      transformation.*
 
 ### Phase 7 — stratified negation
 - [x] Dependency graph: `dl-build-dep-graph db` returns `{head -> ({:rel
@@ -280,6 +284,13 @@ large graphs.
 
 _Newest first._
 
+- 2026-05-08 — Phase 6 stub: `dl-set-strategy! db strategy` and
+  `dl-get-strategy db` user-facing hooks. Default `:semi-naive`;
+  `:magic` is accepted but the actual transformation is deferred,
+  so saturation still uses semi-naive. Lets us tick the
+  "Optional pass — guarded behind dl-set-strategy!" Phase 6 box.
+  3 new eval tests.
+
 - 2026-05-08 — Demo: weighted-DAG shortest path. `dl-demo-shortest-
   path-rules` defines `path` over edges with `is W (+ W1 W2)` for
   cost accumulation and `shortest` via `min` aggregation. 3 demo