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/parser.sx

@@ -366,6 +366,7 @@
       "list"      (str "(" (join " " (map sx-serialize val)) ")")
       "dict"      (sx-serialize-dict val)
       "sx-expr"   (sx-expr-source val)
+      "spread"    (str "(make-spread " (sx-serialize-dict (spread-attrs val)) ")")
       :else       (str val))))