Fixes #44.

Problem

Top-level <DataGrid> never computed a real total count. It only set the count lazily, when the current page returned fewer rows than the page size (itemCount < queryLimit) — i.e. only on a partial last page. On any list spanning more than one full page, paging.info.totalCount / totalPages stayed null, so the UI could not show a row count or "page X of Y", and "jump to last page" was permanently disabled (paging.last() early-returns on totalPages === null). When the total was an exact multiple of the page size, the count was never learned even by paging to the end.

Approach

Mirrors how Contember's own react-dataview solves it (useDataViewTotalCount): issue a standalone COUNT query keyed only on the filter — not on limit/offset — so paging through the list does not recompute it, and refresh() can force a recount.

• Adapter layer — new count query type (CountQuery / CountQueryResult).
  • MockAdapter: counts the filtered set before pagination.
  • ContemberAdapter: paginate<Entity> { pageInfo { totalCount } }, batched into the same request as the sibling list query.
• useEntityCount(entity, { filter, refreshToken }) — new reusable hook in @contember/bindx-react that enqueues the count through the existing batcher.
• Wiring — DataGrid, useDataView and SelectDataView now feed the count into paging.setTotalCount(...). The original partial-page heuristic is kept as a fallback until the count resolves (and for adapters that don't implement count queries).
• refreshTotalCount() now drives a recount via a paging refresh token (PagingStateResult.totalCountRefreshToken) instead of merely nulling the displayed total.

Tests

• tests/react/dataview/dataGridPaginationTotalCount.test.tsx — the issue's reproduction (multi-page list exposes totalCount / totalPages; last-page jump enabled), now passing.
• tests/react/dataview/dataGridCountQuery.test.tsx — total reflects the active filter; last-page jump lands on the right rows; count is keyed on the filter, so paging does not re-issue it (verified with a spy adapter).
• tests/unit/adapter/countQuery.test.ts — MockAdapter count query (no filter / filtered / unknown entity / independent of limit/offset).

All new tests pass; bun run build and bun run typecheck are clean. The 10 pre-existing failures on main (browser/playground "(unnamed)" cases + one form dirty-tracking test) are unrelated and unchanged by this PR.