Expose the record count on the on_query_change payload

[mbaertschi]

Expose the record count on the on_query_change payload

Why

A collection knows how many records match the current filters — it renders the
number in its own pagination footer. A parent LiveView cannot get at it. There
is no callback for it, and page lives on the LiveComponent's assigns.

So an app that wants to show the total anywhere else — a page header, a summary
line, a tab label — has to count the filter a second time:

def handle_info({:query_changed, %{query: query}}, socket) do
  count = query |> Ash.Query.unset([:load, :sort, :limit, :offset]) |> Ash.count!(actor: actor)
  {:noreply, assign(socket, :total, count)}
end

That is a full second COUNT over the same filter, and it's redundant:
QueryBuilder.execute/2 already passes count: true for both offset
(query_builder.ex:277) and keyset (:295) pagination, so Ash has computed
the number milliseconds earlier. The parent also has to dedupe the recount
itself, because on_query_change fires on paging and sorting too, neither of
which can change a count.

What

on_query_change now sends count alongside query:

{event_name, %{query: query, count: 431, id: "users-table"}}

def handle_info({:query_changed, %{count: count, id: "users-table"}}, socket) do
  {:noreply, assign(socket, :total, count)}
end

The value is read off the page that was just loaded, so it costs nothing and
always agrees with the pagination footer — it lands in the same render as the
rows, with no second query and nothing for the parent to dedupe.

• %Ash.Page.Offset{} / %Ash.Page.Keyset{} → the count Ash returned, which
  may be nil if the read produced none. This keeps working if counting ever
  becomes opt-out, as suggested in Feature request: Optional count with keyset pagination #154.
• An unpaginated read (execute_without_pagination/2 returns %{results: ...})
  → length(results), which is the true total in that case since every matching
  record was loaded.
• Anything else → nil.

Compatibility

Additive. The payload is a map, and existing handlers match on query and id,
so a new key breaks no consumer. Documented on both <Cinder.collection> and
<Cinder.Table.table>.

Docs

docs/advanced.md gains a short "Displaying the total elsewhere" subsection.

usage-rules.md had no on_query_change entry at all, so this adds a "Query
Access" section covering the callback and the count. That is the file coding
agents read, and its silence here is what leads to generated code that runs a
second Ash.count! over a query Cinder has already counted — which is exactly
the code this change removes. Happy to drop that hunk if you would rather keep
the PR to the payload alone.

No CHANGELOG entry — the Unreleased entries look maintainer-curated, with
attribution and PR links. Say the word and I will add one in whatever form you
prefer.

Tests

test/cinder/query_change_notification_test.exs covers the offset, keyset,
nil-count, and unpaginated cases, that the query is still delivered alongside
the count, and that nothing is sent when on_query_change is unset. Five of the
six fail without the change. Full suite green (1493 tests, 6 doctests).

[sevenseacat]

I haven't figured out a nice way to automatically curate the changelog yet 😅 so for now yes, it's manually curated. I'll add an entry for this feature. Thank you so much for adding it! 🙌 ❤️