relations: shape queries (siblings, in/out-degree, undirected connected?) computed in SX + 18 tests

Keep the Datalog ruleset minimal — every dl-query re-saturates, so shape
queries are SX BFS over erel, not extra closures. 110/110.

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` → **92/92** (Phases 1–4 complete)
+`bash lib/relations/conformance.sh` → **110/110** (Phases 1–4 complete + extensions)
 
 ## Ground rules
 
@@ -98,8 +98,27 @@ lib/relations/federation.sx
 - [x] `lib/relations/tests/fed.sx` — federated reachability chains, trust gating,
   revocation
 
+## Extensions (post-roadmap)
+
+- [x] **shape queries** — `siblings` (nodes sharing a parent), `out-degree`/
+  `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`.
+
 ## Progress log
 
+- **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
+  added `sibling`/`uedge`/`ureach` as Datalog rules in the global `relations-rules`;
+  because every `dl-query` re-saturates the whole program, the extra recursive
+  undirected closure taxed EVERY query in EVERY suite and the full run blew past
+  10 min. Reverted the ruleset to the Phase-4 set and compute these in SX instead:
+  siblings = children-of(parents-of(node)) − node; connected? = undirected BFS
+  expanding `relations-related` (children ∪ parents) per frontier with a visited
+  set. No new saturation cost; other suites unaffected. NB: the full 110-test
+  conformance takes several minutes under shared-machine contention (sibling loops)
+  — run with `timeout 1200` in the background; individual suites run in seconds.
 - **Phase 4 — federation** (92/92). Re-derived acl's trust-gate shape (not
   copied). engine.sx now derives the whole engine from an EFFECTIVE relation
   `erel` rather than raw `rel`: `erel(S,D,K) :- rel(S,D,K)` (local, always) and