mk: phase 6 done — fd-fire-store iterates, N-queens FD works

The previous fd-fire-store fired every constraint exactly once. That
left the propagation incomplete in chains like
  fd-plus c4 1 a; fd-neq c3 a
where, on the round c4 binds, fd-plus binds a, but fd-neq c3 a was
already past — so the conflict went undetected.

New: fd-store-signature is sum-of-domain-sizes + count-of-bindings.
fd-fire-store calls fd-fire-list and recurses while the signature
strictly decreases. Reaches a fixed point or fails.

This makes N-queens via FD tractable:
  4-queens -> ((2 4 1 3) (3 1 4 2)) — exactly the two solutions.
  5-queens -> 10 solutions (the canonical count), in seconds.

Phase 6 marked complete in the plan: domains, fd-in, fd-eq, fd-neq,
fd-lt, fd-lte, fd-plus, fd-times, fd-distinct, fd-label, all wired
through the constraint-reactivation loop.

Two new tests, 626/626 cumulative.

lib/minikanren/clpfd.sx

@@ -146,11 +146,26 @@
           ((s2 ((first cs) s)))
           (cond ((= s2 nil) nil) (:else (fd-fire-list (rest cs) s2))))))))
 
+(define
+  fd-store-signature
+  (fn
+    (s)
+    (let
+      ((doms (fd-domains-of s)))
+      (let
+        ((dom-sizes (reduce (fn (acc k) (+ acc (len (get doms k)))) 0 (keys doms))))
+        (+ dom-sizes (len (keys s)))))))
+
 (define
   fd-fire-store
   (fn
     (s)
-    (let ((cs (get (fd-store-of s) :constraints))) (fd-fire-list cs s))))
+    (let
+      ((s2 (fd-fire-list (get (fd-store-of s) :constraints) s)))
+      (cond
+        ((= s2 nil) nil)
+        ((= (fd-store-signature s) (fd-store-signature s2)) s2)
+        (:else (fd-fire-store s2))))))
 
 ;; --- user-facing goals ---