Send all responses as sexp wire format with client-side rendering

- Server sends sexp source text, client (sexp.js) renders everything
- SexpExpr marker class for nested sexp composition in serialize()
- sexp_page() HTML shell with data-mount="body" for full page loads
- sexp_response() returns text/sexp for OOB/partial responses
- ~app-body layout component replaces ~app-layout (no raw!)
- ~rich-text is the only component using raw! (for CMS HTML content)
- Fragment endpoints return text/sexp, auto-wrapped in SexpExpr
- All _*_html() helpers converted to _*_sexp() returning sexp source
- Head auto-hoist: sexp.js moves meta/title/link/script[ld+json]
  from rendered body to document.head automatically
- Unknown components render warning box instead of crashing page
- Component kwargs preserve AST for lazy rendering (fixes <> in kwargs)
- Fix unterminated paren in events/sexp/tickets.sexpr

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

cart/sexp/checkout.sexpr

@@ -8,13 +8,13 @@
 
 (defcomp ~cart-checkout-error-order-id (&key order-id)
   (p :class "text-xs text-rose-800/80"
-    "Order ID: " (span :class "font-mono" (raw! order-id))))
+    "Order ID: " (span :class "font-mono" order-id)))
 
-(defcomp ~cart-checkout-error-content (&key error-msg order-html back-url)
+(defcomp ~cart-checkout-error-content (&key error-msg order back-url)
   (div :class "max-w-full px-3 py-3 space-y-4"
     (div :class "rounded-2xl border border-rose-200 bg-rose-50/80 p-4 sm:p-6 text-sm text-rose-900 space-y-2"
       (p :class "font-medium" "Something went wrong.")
-      (p (raw! error-msg))
-      (raw! order-html))
+      (p error-msg)
+      order)
     (div (a :href back-url :class "inline-flex items-center px-3 py-2 text-xs sm:text-sm rounded-full border border-stone-300 bg-white hover:bg-stone-50 transition"
       (i :class "fa fa-shopping-cart mr-2" :aria-hidden "true") "Back to cart"))))