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>

shared/infrastructure/http_utils.py

@@ -14,12 +14,12 @@ from shared.utils import host_url
 
 def vary(resp):
     """
-    Ensure HX-Request and X-Origin are part of the Vary header
-    so caches distinguish HTMX from full-page requests.
+    Ensure SX-Request/HX-Request and X-Origin are part of the Vary header
+    so caches distinguish fragment from full-page requests.
     """
     v = resp.headers.get("Vary", "")
     parts = [p.strip() for p in v.split(",") if p.strip()]
-    for h in ("HX-Request", "X-Origin"):
+    for h in ("SX-Request", "HX-Request", "X-Origin"):
         if h not in parts:
             parts.append(h)
     if parts: