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/bp/cart/page_routes.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 from quart import Blueprint, g, redirect, make_response, url_for
 
 from shared.browser.app.utils.htmx import is_htmx_request
+from shared.sexp.helpers import sexp_response
 from shared.infrastructure.actions import call_action
 from .services import (
     total,
@@ -49,12 +50,13 @@ def register(url_prefix: str) -> Blueprint:
                 ctx, post, cart, cal_entries, page_tickets,
                 ticket_groups, total, calendar_total, ticket_total,
             )
+            return await make_response(html)
         else:
-            html = await render_page_cart_oob(
+            sexp_src = await render_page_cart_oob(
                 ctx, post, cart, cal_entries, page_tickets,
                 ticket_groups, total, calendar_total, ticket_total,
             )
-        return await make_response(html)
+            return sexp_response(sexp_src)
 
     @bp.post("/checkout/")
     async def page_checkout():