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>

blog/sx/handlers/link-card.sx

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

blog/sx/handlers/nav-tree.sx

@@ -1,4 +1,5 @@
 ;; Blog nav-tree fragment handler
+;; returns: sx
 ;;
 ;; Renders the full scrollable navigation menu bar with app icons.
 ;; Uses nav-tree I/O primitive to fetch menu nodes from the blog DB.