mk: phase 5B — project, escape into host SX

(project (vars ...) goal ...) defmacro walks each named var via mk-walk*,
rebinds them in the body's lexical scope, then mk-conjs the body goals on
the same substitution. Hygienic — gensym'd s-param so user vars survive.

Lets you reach into host SX for arithmetic, string ops, anything that
needs a ground value: (project (n) (== q (* n n))), (project (s)
(== q (str s \"!\"))), and so on.

6 new tests, 194/194 cumulative.

plans/minikanren-on-sx.md

@@ -120,8 +120,9 @@ Key semantic mappings:
       `tests/relations.sx`; reverseo/flatteno/permuteo/lengtho deferred)
 
 ### Phase 5 — `project` + `matche` + negation
-- [ ] `project` `(x ...) body` — access reified values of logic vars inside a goal;
-      escapes to ground values for arithmetic or string ops
+- [x] `project` `(x ...) body` — defmacro: rebinds named vars to `(mk-walk* var s)`
+      in the body's lexical scope, then runs `(mk-conj body...)` on the same
+      substitution. Hygienic via gensym'd `s`-param. (`Phase 5 piece B`)
 - [ ] `matche` — pattern matching over logic terms (extension from core.logic)
       `(matche l ((head . tail) goal) (() goal))`
 - [x] `conda` — soft-cut: first non-failing head wins; ALL of head's answers
@@ -156,6 +157,10 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-07** — **Phase 5 piece B — project**: `lib/minikanren/project.sx` —
+  defmacro that walks each named var, rebinds them, and runs the body's mk-conj.
+  Demonstrated escape into host arithmetic / string ops (`(* n n)`, `(str s "!")`).
+  Hygienic gensym'd s-param. 6 new tests, 194/194 cumulative.
 - **2026-05-07** — **Peano arithmetic** (`lib/minikanren/peano.sx`): zeroo, pluso,
   minuso, lteo, lto, *o on Peano-encoded naturals (`:z` / `(:s n)`). pluso runs
   forward, backward, and enumerates: `(run* q (fresh (a b) (pluso a b 3)