Fix spread serialization in aser/async-aser wire format

Spread values from make-spread were crashing the wire format serializer:
- serialize() had no "spread" case, fell through to (str val) producing
  Python repr "<shared.sx.ref.sx_ref._Spread...>" which was treated as
  an undefined symbol
- aser-call/async-aser-call didn't handle spread children — now merges
  spread attrs as keyword args into the parent element
- aser-fragment/async-aser-fragment didn't filter spreads — now filters
  them (fragments have no parent element to merge into)
- serialize() now handles spread type: (make-spread {:key "val"})

Added 3 aser-spreads tests. All 562 tests pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/adapter-async.sx

@@ -775,6 +775,7 @@
 
 (define-async async-aser-fragment :effects [render io]
   (fn ((children :as list) (env :as dict) ctx)
+    ;; Spreads are filtered — fragments have no parent element to merge into
     (let ((parts (list)))
       (for-each
         (fn (c)
@@ -782,10 +783,10 @@
             (if (= (type-of result) "list")
               (for-each
                 (fn (item)
-                  (when (not (nil? item))
+                  (when (and (not (nil? item)) (not (spread? item)))
                     (append! parts (serialize item))))
                 result)
-              (when (not (nil? result))
+              (when (and (not (nil? result)) (not (spread? result)))
                 (append! parts (serialize result))))))
         children)
       (if (empty? parts)
@@ -885,13 +886,21 @@
                 (set! i (inc i)))
               (let ((result (async-aser arg env ctx)))
                 (when (not (nil? result))
-                  (if (= (type-of result) "list")
+                  (if (spread? result)
+                    ;; Spread child — merge attrs as keyword args into parent element
                     (for-each
-                      (fn (item)
-                        (when (not (nil? item))
-                          (append! parts (serialize item))))
-                      result)
-                    (append! parts (serialize result))))
+                      (fn (k)
+                        (let ((v (dict-get (spread-attrs result) k)))
+                          (append! parts (str ":" k))
+                          (append! parts (serialize v))))
+                      (keys (spread-attrs result)))
+                    (if (= (type-of result) "list")
+                      (for-each
+                        (fn (item)
+                          (when (not (nil? item))
+                            (append! parts (serialize item))))
+                        result)
+                      (append! parts (serialize result)))))
                 (set! i (inc i))))))
         args)
       (when token (svg-context-reset! token))