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/fragments.py

@@ -76,15 +76,18 @@ async def fetch_fragment(
     timeout: float = _DEFAULT_TIMEOUT,
     required: bool = True,
 ) -> str:
-    """Fetch an HTML fragment from another app.
+    """Fetch a fragment from another app.
 
-    Returns the raw HTML string.  When *required* is True (default),
-    raises ``FragmentError`` on network errors or non-200 responses.
+    Returns an HTML string or a ``SexpExpr`` (when the provider responds
+    with ``text/sexp``).  When *required* is True (default), raises
+    ``FragmentError`` on network errors or non-200 responses.
     When *required* is False, returns ``""`` on failure.
 
     Automatically returns ``""`` when called inside a fragment request
     to prevent circular dependencies between apps.
     """
+    from shared.sexp.parser import SexpExpr
+
     if _is_fragment_request():
         return ""
 
@@ -98,6 +101,9 @@ async def fetch_fragment(
             timeout=timeout,
         )
         if resp.status_code == 200:
+            ct = resp.headers.get("content-type", "")
+            if "text/sexp" in ct:
+                return SexpExpr(resp.text)
             return resp.text
         msg = f"Fragment {app_name}/{fragment_type} returned {resp.status_code}"
         if required: