Fix orders defpage: length→len primitive, handle _RawHTML in serialize()

- Fix undefined symbol 'length' → use 'len' primitive in orders.sx
- Add _RawHTML handling in serialize() — wraps as (raw! "...") for SX wire format
  instead of falling through to repr() which produced unparseable symbol names

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

shared/sx/parser.py

@@ -336,6 +336,17 @@ def serialize(expr: Any, indent: int = 0, pretty: bool = False) -> str:
             items.append(serialize(v, indent, pretty))
         return "{" + " ".join(items) + "}"
 
+    # _RawHTML — pre-rendered HTML; wrap as (raw! "...") for SX wire format
+    from .html import _RawHTML
+    if isinstance(expr, _RawHTML):
+        escaped = (
+            expr.html.replace("\\", "\\\\")
+            .replace('"', '\\"')
+            .replace("\n", "\\n")
+            .replace("</script", "<\\/script")
+        )
+        return f'(raw! "{escaped}")'
+
     # Catch callables (Python functions leaked into sx data)
     if callable(expr):
         import logging