HS pick runtime: guard nil inputs so pick first/last/items/match/matches don't hang

- hs-pick-first/last/random/items/slice: short-circuit nil or non-list
  (strings flow through unchanged).
- New hs-pick-match / hs-pick-matches wrappers around regex-match /
  regex-find-all, also nil-safe; compiler routes pick-match / pick-matches
  through them. Unblocks 'pick first from null returns null' and
  'pick match from null returns null' which previously looped past
  step_limit.

shared/static/wasm/sx/hs-compiler.sx

@@ -818,7 +818,7 @@
                   (quote set!)
                   (quote it)
                   (list
-                    (quote regex-match)
+                    (quote hs-pick-match)
                     (hs-to-sx (nth ast 1))
                     (hs-to-sx (nth ast 2)))))
               ((= head (quote pick-matches))
@@ -826,7 +826,7 @@
                   (quote set!)
                   (quote it)
                   (list
-                    (quote regex-find-all)
+                    (quote hs-pick-matches)
                     (hs-to-sx (nth ast 1))
                     (hs-to-sx (nth ast 2)))))
               ((= head (quote prop-is))

shared/static/wasm/sx/hs-runtime.sx

@@ -1767,37 +1767,67 @@
   hs-slice
   (fn
     (col start end)
-    (let
-      ((s (if (nil? start) 0 start))
-        (e (if (nil? end) (len col) (+ end 1))))
-      (slice col s e))))
+    (cond
+      ((nil? col) nil)
+      ((not (list? col)) col)
+      (true
+        (let
+          ((s (if (nil? start) 0 start))
+            (e (if (nil? end) (len col) (+ end 1))))
+          (slice col s e))))))
 
 (define
   hs-pick-first
   (fn
     (col n)
-    (let ((m (if (< n (len col)) n (len col)))) (slice col 0 m))))
+    (cond
+      ((nil? col) nil)
+      ((not (list? col)) col)
+      (true
+        (let ((m (if (< n (len col)) n (len col)))) (slice col 0 m))))))
 
 (define
   hs-pick-last
   (fn
     (col n)
-    (let
-      ((total (len col)))
-      (let
-        ((start (if (< n total) (- total n) 0)))
-        (slice col start total)))))
+    (cond
+      ((nil? col) nil)
+      ((not (list? col)) col)
+      (true
+        (let
+          ((total (len col)))
+          (let
+            ((start (if (< n total) (- total n) 0)))
+            (slice col start total)))))))
 
 (define
   hs-pick-random
   (fn
     (col n)
-    (if
-      (nil? n)
-      (first col)
-      (let ((m (if (< n (len col)) n (len col)))) (slice col 0 m)))))
+    (cond
+      ((nil? col) nil)
+      ((not (list? col)) col)
+      ((nil? n) (first col))
+      (true
+        (let ((m (if (< n (len col)) n (len col)))) (slice col 0 m))))))
 
-(define hs-pick-items (fn (col start end) (slice col start end)))
+(define
+  hs-pick-items
+  (fn
+    (col start end)
+    (cond
+      ((nil? col) nil)
+      ((not (list? col)) col)
+      (true (slice col start end)))))
+
+(define
+  hs-pick-match
+  (fn
+    (regex haystack)
+    (cond
+      ((nil? haystack) nil)
+      ((nil? regex) nil)
+      (true (regex-match regex haystack)))))
 
 (define
   hs-sorted-by