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/test-aser.sx

@@ -270,3 +270,21 @@
   (deftest "or returns last falsy"
     (assert-equal "false"
       (render-sx "(or false false)"))))
+
+
+;; --------------------------------------------------------------------------
+;; Spread serialization
+;; --------------------------------------------------------------------------
+
+(defsuite "aser-spreads"
+  (deftest "spread in element merges attrs"
+    (assert-equal "(div :class \"card\" \"hello\")"
+      (render-sx "(div (make-spread {:class \"card\"}) \"hello\")")))
+
+  (deftest "multiple spreads merge into element"
+    (assert-equal "(div :class \"card\" :style \"color:red\" \"hello\")"
+      (render-sx "(div (make-spread {:class \"card\"}) (make-spread {:style \"color:red\"}) \"hello\")")))
+
+  (deftest "spread in fragment is filtered"
+    (assert-equal "(<> \"hello\")"
+      (render-sx "(<> (make-spread {:class \"card\"}) \"hello\")"))))