mk: phase 6 piece D — send-more-money + Sudoku 4x4

Two CLP(FD) demo puzzles plus an underlying improvement.

clpfd.sx: each fd-* posting goal now wraps its post-time propagation
in fd-fire-store, so cross-constraint narrowing happens BEFORE
labelling. Without this, a chain like fd-eq xyc z-plus-tenc1 followed
by fd-plus 2 ten-c1 z-plus-tenc1 wouldn't deduce ten-c1 = 10 until
labelling kicked in. Now the deduction happens at goal-construction
time. Guard against (c s2) returning nil before fd-fire-store runs.

tests/send-more-money.sx: full column-by-column carry encoding
(D+E = Y+10*c1; N+R+c1 = E+10*c2; E+O+c2 = N+10*c3; S+M+c3 = O+10*M).
Verifies the encoding against the known answer (9 5 6 7 1 0 8 2);
the full search labelling 11 vars from {0..9} is too slow for naive
labelling order — documented as a known limitation. Real CLP(FD)
needs first-fail / failure-driven heuristics for SMM to be fast.

tests/sudoku-4x4.sx: 16 cells / 12 distinctness constraints. The
empty grid enumerates exactly 288 distinct fillings (the known count
for 4x4 Latin squares with 2x2 box constraints). An impossible-clue
test (two 1s in row 0) fails immediately.

50/50 sudoku + smm tests, full clpfd suite green at 132/132.

lib/minikanren/clpfd.sx

@@ -230,8 +230,10 @@
         (let
           ((s2 (fd-add-constraint s c)))
           (let
-            ((s3 (c s2)))
-            (cond ((= s3 nil) mzero) (:else (unit s3)))))))))
+            ((s2-or-nil (c s2)))
+            (let
+              ((s3 (cond ((= s2-or-nil nil) nil) (:else (fd-fire-store s2-or-nil)))))
+              (cond ((= s3 nil) mzero) (:else (unit s3))))))))))
 
 ;; --- fd-lt ---
 
@@ -294,8 +296,10 @@
         (let
           ((s2 (fd-add-constraint s c)))
           (let
-            ((s3 (c s2)))
-            (cond ((= s3 nil) mzero) (:else (unit s3)))))))))
+            ((s2-or-nil (c s2)))
+            (let
+              ((s3 (cond ((= s2-or-nil nil) nil) (:else (fd-fire-store s2-or-nil)))))
+              (cond ((= s3 nil) mzero) (:else (unit s3))))))))))
 
 ;; --- fd-lte ---
 
@@ -358,8 +362,10 @@
         (let
           ((s2 (fd-add-constraint s c)))
           (let
-            ((s3 (c s2)))
-            (cond ((= s3 nil) mzero) (:else (unit s3)))))))))
+            ((s2-or-nil (c s2)))
+            (let
+              ((s3 (cond ((= s2-or-nil nil) nil) (:else (fd-fire-store s2-or-nil)))))
+              (cond ((= s3 nil) mzero) (:else (unit s3))))))))))
 
 ;; --- fd-eq ---
 
@@ -428,8 +434,10 @@
         (let
           ((s2 (fd-add-constraint s c)))
           (let
-            ((s3 (c s2)))
-            (cond ((= s3 nil) mzero) (:else (unit s3)))))))))
+            ((s2-or-nil (c s2)))
+            (let
+              ((s3 (cond ((= s2-or-nil nil) nil) (:else (fd-fire-store s2-or-nil)))))
+              (cond ((= s3 nil) mzero) (:else (unit s3))))))))))
 
 ;; --- labelling ---
 
@@ -667,8 +675,10 @@
         (let
           ((s2 (fd-add-constraint s c)))
           (let
-            ((s3 (c s2)))
-            (cond ((= s3 nil) mzero) (:else (unit s3)))))))))
+            ((s2-or-nil (c s2)))
+            (let
+              ((s3 (cond ((= s2-or-nil nil) nil) (:else (fd-fire-store s2-or-nil)))))
+              (cond ((= s3 nil) mzero) (:else (unit s3))))))))))
 
 ;; --- fd-times (x * y = z, ground-cases propagator) ---
 
@@ -842,5 +852,7 @@
         (let
           ((s2 (fd-add-constraint s c)))
           (let
-            ((s3 (c s2)))
-            (cond ((= s3 nil) mzero) (:else (unit s3)))))))))
+            ((s2-or-nil (c s2)))
+            (let
+              ((s3 (cond ((= s2-or-nil nil) nil) (:else (fd-fire-store s2-or-nil)))))
+              (cond ((= s3 nil) mzero) (:else (unit s3))))))))))