mk: phase 2B — fresh, defmacro form + call-fresh

(fresh (x y z) g1 g2 ...) expands to a let that calls (make-var) for each
named var, then mk-conjs the goals. call-fresh is the function-shaped
alternative for programmatic goal building.

9 new tests: empty-vars, single var, multi-var multi-goal, fresh under
disj, nested fresh, call-fresh equivalents. 91/91 cumulative.

plans/minikanren-on-sx.md

@@ -72,7 +72,9 @@ Key semantic mappings:
 - [x] `conj2` / `mk-conj` (variadic) — sequential conjunction
 - [x] `disj2` / `mk-disj` (variadic) — interleaved disjunction (raw — `conde`
       adds the implicit-conj-per-clause sugar later)
-- [ ] `fresh` — introduces logic variables inside a goal body
+- [x] `fresh` — introduces logic variables inside a goal body. Implemented as a
+      defmacro: `(fresh (x y) g1 g2 ...)` ⇒ `(let ((x (make-var)) (y (make-var)))
+      (mk-conj g1 g2 ...))`. Also `call-fresh` for programmatic goal building.
 - [ ] `conde` — sugar over disj+conj, one row per clause
 - [ ] `condu` — committed choice (soft-cut)
 - [ ] `onceo` — succeeds at most once
@@ -141,6 +143,9 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-07** — **Phase 2 piece B** (`fresh`): `lib/minikanren/fresh.sx` (~10 lines).
+  defmacro form for nice user-facing syntax + `call-fresh` for programmatic use.
+  9 new tests in `tests/fresh.sx`, 91/91 cumulative.
 - **2026-05-07** — **Phase 2 piece A** (streams + ==/conj/disj): `lib/minikanren/stream.sx`
   (mzero/unit/mk-mplus/mk-bind/stream-take, ~25 lines of code) + `lib/minikanren/goals.sx`
   (succeed/fail/==/==-check/conj2/disj2/mk-conj/mk-disj, ~30 lines). Found and noted