mk: numbero / stringo / symbolo type predicates

Ground-only type tests via project. Each succeeds iff its argument
walks to the corresponding host value type. Composes with membero for
type-filtered enumeration:

  (fresh (x) (membero x (list 1 "a" 2 "b" 3)) (numbero x) (== q x))
    -> (1 2 3)

12 new tests, 328/328 cumulative. Caveat: SX keywords are strings, so
(stringo :k) succeeds.

plans/minikanren-on-sx.md

@@ -173,6 +173,11 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **numbero / stringo / symbolo (type predicates)**: ground-only
+  type tests via project. Compose with `membero` for type-filtered enumeration:
+  `(fresh (x) (membero x (1 "a" 2 "b" 3)) (numbero x) (== q x))` → `(1 2 3)`.
+  Note: SX keywords are strings, so `(stringo :k)` succeeds. 12 new tests,
+  328/328 cumulative.
 - **2026-05-08** — **Graph reachability via patho**: classic miniKanren
   graph search. `edgeo` looks up edges in a fact list via `membero`; `patho`
   recursively builds paths via direct-edge OR (one edge + recurse + cons).