[ADD] website_sale_lazy_loading: load the /shop listing on demand

The shop pages products: every page is a full reload that throws away the
grid the visitor was reading. This appends the next page to the grid
instead, on scroll or on a "Load more products" click, per website.

The controller does not duplicate /shop. The new /shop/lazy_products calls
shop() and renders the cards out of the qcontext it prepared, so search,
categories, attributes, tags, price filter, sort order and pricelists are
supported by construction -- and so are the values other modules add to the
listing, the wishlist state among them. Out of range pages answer empty
rather than the last page again, which portal.pager would otherwise clamp
to and the frontend would append as duplicates.

The product loop of website_sale.products is replaced by a call to a shared
template, so the first page and the appended ones are the same markup: a
ribbon, a price or a button another module adds to products_item shows up
on every card, not only on the ones the initial render produced.

Progressive enhancement throughout: the first page and the pager are still
what the standard controller renders, and the pager is only hidden once the
widget is running. Without JavaScript -- and for crawlers -- the shop is
exactly what it is without this module. The page size is the shop layout's
"Products per page", the value the pager already uses, so there is nothing
to keep in sync.

The frontend takes no decision about what to show: the server sends the
mode, the URL of the listing on screen and each page of cards. It observes
a block below the grid rather than listening to scroll, restarts the public
widgets on the appended cards, and keeps a button as the fallback for a
failed request or a browser without IntersectionObserver.

Tests cover the block rendering per mode, the filters travelling in the
AJAX URL, and the endpoint on a next page, the last page, an out of range
page, a broken page number and an unknown category.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

website_sale_lazy_loading/readme/USAGE.rst

@@ -0,0 +1,24 @@
+There is nothing to do at runtime: once the mode is set, ``/shop`` loads its
+pages on demand.
+
+How it works
+~~~~~~~~~~~~
+
+``/shop`` is rendered by the standard controller, with its pager. When a next
+page exists, this module adds a block below the grid carrying the loading mode
+and the URL of the current listing (search, category, attributes, tags, price
+range and sort order included). The frontend widget then hides the pager and
+requests ``/shop/lazy_products?...&page=N``, which answers with the product
+cards of that page as an HTML fragment plus whether a further page exists, and
+appends them to the grid.
+
+Consequences worth knowing:
+
+* Product links keep the page they belong to (``?page=3``), so coming back from
+  a product page lands on the right part of the listing.
+* Changing a filter, a category or the sort order is still a normal page load;
+  lazy loading restarts from the first page of the new listing.
+* Reloading the page starts again from the first page: the appended pages are
+  not part of the URL.
+* The endpoint answers with the same data ``/shop`` would render for that page,
+  under the rights of the visitor making the request.