Make SxExpr a str subclass, sx_call/render functions return SxExpr

SxExpr is now a str subclass so it works everywhere a plain string
does (join, isinstance, f-strings) while serialize() still emits it
unquoted. sx_call() and all internal render functions (_render_to_sx,
async_eval_to_sx, etc.) return SxExpr, eliminating the "forgot to
wrap" bug class that caused the sx_content leak and list serialization
bugs.

- Phase 0: SxExpr(str) with .source property, __add__/__radd__
- Phase 1: sx_call returns SxExpr (drop-in, all 200+ sites unchanged)
- Phase 2: async_eval_to_sx, async_eval_slot_to_sx, _render_to_sx,
  mobile_menu_sx return SxExpr; remove isinstance(str) workaround
- Phase 3: Remove ~150 redundant SxExpr() wrappings across 45 files
- Phase 4: serialize() docstring, handler return docs, ;; returns: sx

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

shared/sx/async_eval.py

@@ -1010,10 +1010,10 @@ async def async_eval_to_sx(
         ctx = RequestContext()
     result = await _aser(expr, env, ctx)
     if isinstance(result, SxExpr):
-        return result.source
+        return result
     if result is None or result is NIL:
-        return ""
-    return serialize(result)
+        return SxExpr("")
+    return SxExpr(serialize(result))
 
 
 async def async_eval_slot_to_sx(
@@ -1039,10 +1039,10 @@ async def async_eval_slot_to_sx(
             if isinstance(comp, Component):
                 result = await _aser_component(comp, expr[1:], env, ctx)
                 if isinstance(result, SxExpr):
-                    return result.source
+                    return result
                 if result is None or result is NIL:
-                    return ""
-                return serialize(result)
+                    return SxExpr("")
+                return SxExpr(serialize(result))
             else:
                 import logging
                 logging.getLogger("sx.eval").error(
@@ -1056,14 +1056,10 @@ async def async_eval_slot_to_sx(
     # Fall back to normal async_eval_to_sx
     result = await _aser(expr, env, ctx)
     if isinstance(result, SxExpr):
-        return result.source
-    if result is None or result is NIL:
-        return ""
-    # Page helpers return SX source strings from render_to_sx() —
-    # pass through directly instead of quoting via serialize().
-    if isinstance(result, str):
         return result
-    return serialize(result)
+    if result is None or result is NIL:
+        return SxExpr("")
+    return SxExpr(serialize(result))
 
 
 async def _aser(expr: Any, env: dict[str, Any], ctx: RequestContext) -> Any:
@@ -1071,10 +1067,10 @@ async def _aser(expr: Any, env: dict[str, Any], ctx: RequestContext) -> Any:
     for everything else."""
     if isinstance(expr, (int, float, bool)):
         return expr
-    if isinstance(expr, str):
-        return expr
     if isinstance(expr, SxExpr):
         return expr
+    if isinstance(expr, str):
+        return expr
     if expr is None or expr is NIL:
         return NIL