relations: Phase 1 schema + direct relations (rel facts, relate/unrelate, children/parents/related) + 22 tests

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

lib/relations/schema.sx

@@ -0,0 +1,40 @@
+;; lib/relations/schema.sx — relationship fact vocabulary over lib/datalog/.
+;;
+;; relations is content-agnostic: a node is an opaque id (a symbol or string);
+;; domains own what ids mean. A relationship is a single Datalog fact
+;;
+;;     rel(Src, Dst, Kind)
+;;
+;; meaning "Src is related to Dst under Kind" (read directionally: Src is the
+;; parent/owner/origin, Dst the child/member/reply). Kind is an open vocabulary;
+;; the names below are the platform's well-known kinds but relate accepts any
+;; kind symbol — Datalog is untyped and domains may coin their own.
+
+(define relations-kinds (quote (parent member reply variant origin link)))
+
+(define relations-rel (fn (src dst kind) (list (quote rel) src dst kind)))
+
+(define relations-rel-src (fn (f) (nth f 1)))
+
+(define relations-rel-dst (fn (f) (nth f 2)))
+
+(define relations-rel-kind (fn (f) (nth f 3)))
+
+(define
+  relations-member?
+  (fn
+    (x xs)
+    (cond
+      ((= (len xs) 0) false)
+      ((= (first xs) x) true)
+      (else (relations-member? x (rest xs))))))
+
+(define
+  relations-known-kind?
+  (fn (k) (relations-member? k relations-kinds)))
+
+(define
+  relations-fact-valid?
+  (fn
+    (f)
+    (and (list? f) (= (len f) 4) (= (first f) (quote rel)))))