relations: tree/DAG queries (common-ancestors, lca, topo-order) in SX + 16 tests

lib/relations/tree.sx over reach/ancestors/rnode — no new Datalog closures. 126/126.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/relations-on-sx.md

@@ -18,7 +18,7 @@ links. Reuses `lib/datalog/` — does not reimplement the engine.
 
 ## Status (rolling)
 
-`bash lib/relations/conformance.sh` → **110/110** (Phases 1–4 complete + extensions)
+`bash lib/relations/conformance.sh` → **126/126** (Phases 1–4 complete + extensions)
 
 ## Ground rules
 
@@ -104,9 +104,23 @@ lib/relations/federation.sx
   `in-degree`, weakly-connected `connected?` (undirected reachability). Computed in
   SX over the fast direct `erel` queries (BFS) — deliberately NOT added as Datalog
   closures, to keep the per-query saturation cheap. `lib/relations/tests/shape.sx`.
+- [x] **tree/DAG queries** — `common-ancestors` (ancestor-set intersection), `lca`
+  (lowest common ancestors — a set; tree → singleton, DAG → may be several),
+  `topo-order` (Kahn-style; nil for cyclic kinds). New `lib/relations/tree.sx`,
+  computed in SX over `reach`/`ancestors`/`rnode`. `lib/relations/tests/tree.sx`.
 
 ## Progress log
 
+- **Extension: tree/DAG queries** (126/126). New `lib/relations/tree.sx`:
+  `relations-common-ancestors` (intersection of the two ancestor sets),
+  `relations-lca` (common ancestors with no other common ancestor reachable below
+  them — a SET, since a DAG can have several lowest common ancestors; a tree gives
+  one), `relations-topo-order` (Kahn-style level-by-level: place every node whose
+  parents are all placed; nil for a cyclic kind) + `relations-nodes` (the `rnode`
+  set) and `relations/...` wrappers. All in SX over the engine's fast queries —
+  again no new Datalog closures. `tree.sx` (16 tests) covers diamond common
+  ancestors, LCA on tree vs converging-DAG, no-common-ancestor, topo validity
+  (parents precede children), and cyclic-kind → nil.
 - **Extension: shape queries** (110/110). Added `relations-siblings`,
   `relations-out-degree`/`-in-degree`, `relations-connected?` (+ `relations/...`
   current-db wrappers) and `shape.sx` (18 tests). Design note: an earlier attempt