mk: relational database queries — Datalog-style demo

tests/rdb.sx shows the library as a small Datalog engine over fact
tables. Each table is an SX list of tuples, wrapped by a relation that
does (membero (list ...) table). Queries compose selection, projection,
and joins entirely in run* / fresh / conde / membero / intarith / nafc.

Five queries: dept filter, salary > threshold, employee-project join,
intersection (engineers on a specific project), anyone on multiple
distinct projects.

5 new tests, 336/336 cumulative.

plans/minikanren-on-sx.md

@@ -173,6 +173,12 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **Datalog-style relational database queries**: tests/rdb.sx
+  shows miniKanren as a query engine over fact tables. Defines two tables
+  (employees + project assignments), wraps each with a relation that does
+  membero over the table, then expresses queries: dept filter, salary >
+  threshold (intarith), join engineers ↔ runtime project, find anyone on
+  multiple distinct projects (nafc + ==). 5 new tests, 336/336 cumulative.
 - **2026-05-08** — **Cyclic graph behaviour test (motivates Phase 7 tabling)**:
   Demonstrates that naive patho on a 2-cycle generates ever-longer paths.
   `run 5` truncates to a finite prefix; `run*` would diverge. This is