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/sx/handlers/account-nav-item.sx

@@ -1,4 +1,5 @@
 ;; Events account-nav-item fragment handler
+;; returns: sx
 ;;
 ;; Renders tickets + bookings links for the account dashboard nav.
 

events/sx/handlers/account-page.sx

@@ -1,4 +1,5 @@
 ;; Account-page fragment handler
+;; returns: sx
 ;;
 ;; Renders tickets or bookings panel for the account dashboard.
 ;; slug=tickets → ticket list; slug=bookings → booking list.

events/sx/handlers/container-cards.sx

@@ -1,4 +1,5 @@
 ;; Container-cards fragment handler
+;; returns: sx
 ;;
 ;; Returns HTML with <!-- card-widget:ID --> comment markers so the
 ;; blog consumer can split per-post fragments.  Each post section

events/sx/handlers/container-nav.sx

@@ -1,4 +1,5 @@
 ;; Events container-nav fragment handler
+;; returns: sx
 ;;
 ;; Renders calendar entry nav items + calendar link nav items
 ;; for the scrollable navigation panel on blog post pages.

events/sx/handlers/link-card.sx

@@ -1,4 +1,5 @@
 ;; Events link-card fragment handler
+;; returns: sx
 ;;
 ;; Renders event page preview card(s) by slug.
 ;; Supports single mode (?slug=x) and batch mode (?keys=x,y,z).