Remove render_to_sx from public API: enforce sx_call for all service code

Replace ~250 render_to_sx calls across all services with sync sx_call,
converting many async functions to sync where no other awaits remained.
Make render_to_sx/render_to_sx_with_env private (_render_to_sx).
Add (post-header-ctx) IO primitive and shared post/post-admin defmacros.
Convert built-in post/post-admin layouts from Python to register_sx_layout
with .sx defcomps. Remove dead post_admin_mobile_nav_sx.

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

shared/sx/primitives_io.py

@@ -46,6 +46,7 @@ IO_PRIMITIVES: frozenset[str] = frozenset({
     "url-for",
     "route-prefix",
     "root-header-ctx",
+    "post-header-ctx",
     "select-colours",
     "account-nav-ctx",
     "app-rights",
@@ -482,6 +483,80 @@ async def _io_app_rights(
     return getattr(g, "rights", None) or {}
 
 
+async def _io_post_header_ctx(
+    args: list[Any], kwargs: dict[str, Any], ctx: RequestContext
+) -> dict[str, Any]:
+    """``(post-header-ctx)`` → dict with post-level header values.
+
+    Reads post data from ``g._defpage_ctx`` (set by per-service page
+    helpers), fetches container-nav and page cart count.  Result is
+    cached on ``g`` per request.
+
+    Returns dict with keys: slug, title, feature-image, link-href,
+    container-nav, page-cart-count, cart-href, admin-href, is-admin,
+    is-admin-page, select-colours.
+    """
+    from quart import g, request
+    cached = getattr(g, "_post_header_ctx", None)
+    if cached is not None:
+        return cached
+
+    from shared.infrastructure.urls import app_url
+    from .types import NIL
+    from .parser import SxExpr
+
+    dctx = getattr(g, "_defpage_ctx", None) or {}
+    post = dctx.get("post") or {}
+    slug = post.get("slug", "")
+    if not slug:
+        result: dict[str, Any] = {"slug": ""}
+        g._post_header_ctx = result
+        return result
+
+    title = (post.get("title") or "")[:160]
+    feature_image = post.get("feature_image") or NIL
+
+    # Container nav (pre-fetched by page helper into defpage ctx)
+    raw_nav = dctx.get("container_nav") or ""
+    container_nav: Any = NIL
+    nav_str = str(raw_nav).strip()
+    if nav_str and nav_str.replace("(<>", "").replace(")", "").strip():
+        if isinstance(raw_nav, SxExpr):
+            container_nav = raw_nav
+        else:
+            container_nav = SxExpr(nav_str)
+
+    page_cart_count = dctx.get("page_cart_count", 0) or 0
+
+    rights = getattr(g, "rights", None) or {}
+    is_admin = (
+        rights.get("admin", False)
+        if isinstance(rights, dict)
+        else getattr(rights, "admin", False)
+    )
+
+    is_admin_page = dctx.get("is_admin_section") or "/admin" in request.path
+
+    from quart import current_app
+    select_colours = current_app.jinja_env.globals.get("select_colours", "")
+
+    result = {
+        "slug": slug,
+        "title": title,
+        "feature-image": feature_image,
+        "link-href": app_url("blog", f"/{slug}/"),
+        "container-nav": container_nav,
+        "page-cart-count": page_cart_count,
+        "cart-href": app_url("cart", f"/{slug}/") if page_cart_count else "",
+        "admin-href": app_url("blog", f"/{slug}/admin/"),
+        "is-admin": is_admin,
+        "is-admin-page": is_admin_page or NIL,
+        "select-colours": select_colours,
+    }
+    g._post_header_ctx = result
+    return result
+
+
 _IO_HANDLERS: dict[str, Any] = {
     "frag": _io_frag,
     "query": _io_query,
@@ -499,6 +574,7 @@ _IO_HANDLERS: dict[str, Any] = {
     "url-for": _io_url_for,
     "route-prefix": _io_route_prefix,
     "root-header-ctx": _io_root_header_ctx,
+    "post-header-ctx": _io_post_header_ctx,
     "select-colours": _io_select_colours,
     "account-nav-ctx": _io_account_nav_ctx,
     "app-rights": _io_app_rights,