mk: phase 6G — fd-plus (x + y = z)

Ground-cases propagator: when at least two of {x, y, z} walk to
ground numbers, the third is derived (or checked, if also ground).
Three vars with domains: deferred — no bounds-consistency in this
iteration.

Includes a small fd-bind-or-narrow helper that handles the common
"bind a var to a target int, respecting any existing domain"
pattern shared across propagators.

7 new tests: ground/ground/ground, recover x, recover y, impossible
case, domain-check rejection, x+y=5 enumeration, large numbers.
617/617 cumulative.

lib/minikanren/clpfd.sx

@@ -484,3 +484,53 @@
         (mk-conj
           (fd-distinct-from-head (first vars) (rest vars))
           (fd-distinct (rest vars)))))))
+
+;; --- fd-plus (x + y = z, ground-cases propagator) ---
+
+(define
+  fd-bind-or-narrow
+  (fn
+    (w target s)
+    (cond
+      ((number? w) (cond ((= w target) s) (:else nil)))
+      ((is-var? w)
+        (let
+          ((wd (fd-domain-of s (var-name w))))
+          (cond
+            ((and (not (= wd nil)) (not (fd-dom-member? target wd))) nil)
+            (:else
+              (let
+                ((s2 (mk-unify w target s)))
+                (cond ((= s2 nil) nil) (:else s2)))))))
+      (:else nil))))
+
+(define
+  fd-plus-prop
+  (fn
+    (x y z s)
+    (let
+      ((wx (mk-walk x s)) (wy (mk-walk y s)) (wz (mk-walk z s)))
+      (cond
+        ((and (number? wx) (number? wy) (number? wz))
+          (cond ((= (+ wx wy) wz) s) (:else nil)))
+        ((and (number? wx) (number? wy))
+          (fd-bind-or-narrow wz (+ wx wy) s))
+        ((and (number? wx) (number? wz))
+          (fd-bind-or-narrow wy (- wz wx) s))
+        ((and (number? wy) (number? wz))
+          (fd-bind-or-narrow wx (- wz wy) s))
+        (:else s)))))
+
+(define
+  fd-plus
+  (fn
+    (x y z)
+    (fn
+      (s)
+      (let
+        ((c (fn (sp) (fd-plus-prop x y z sp))))
+        (let
+          ((s2 (fd-add-constraint s c)))
+          (let
+            ((s3 (c s2)))
+            (cond ((= s3 nil) mzero) (:else (unit s3)))))))))