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>

events/sxc/pages/slots.py

@@ -160,7 +160,7 @@ def _slot_header_html(ctx: dict, *, oob: bool = False) -> str:
                                   name=slot.name, description=desc)
 
     return sx_call("menu-row-sx", id="slot-row", level=5,
-                  link_label_content=SxExpr(label_sx),
+                  link_label_content=label_sx,
                   child_id="slot-header-child", oob=oob)
 
 
@@ -201,7 +201,7 @@ def render_slot_main_panel(slot, calendar, *, oob: bool = False) -> str:
 
     result = sx_call("events-slot-panel",
                     slot_id=sid, list_container=list_container,
-                    days=SxExpr(days_html),
+                    days=days_html,
                     flexible="yes" if flexible else "no",
                     time_str=f"{time_start} \u2014 {time_end}",
                     cost_str=cost_str,
@@ -259,7 +259,7 @@ def render_slots_table(slots, calendar) -> str:
                                 pill_cls=pill_cls, hx_select=hx_select,
                                 slot_name=s.name, description=desc,
                                 flexible="yes" if s.flexible else "no",
-                                days=SxExpr(days_html),
+                                days=days_html,
                                 time_str=f"{time_start} - {time_end}",
                                 cost_str=cost_str, action_btn=action_btn,
                                 del_url=del_url,