fix(navigation): keep down arrow visible after letter jump

[wizzomafizzo]

Reproduction

1. Open the View menu.
2. Choose Jump to letter.
3. Select a letter whose target requires paginated loading.
4. Observe the sidebar after results load.

The down arrow disappeared at the end of the currently loaded slice even though more results existed.

Root cause

Letter jumps can land on the final currently loaded page while the model still has an unknown authoritative total and reports hasMorePages. The sidebar derived its arrow only from loaded pageCount, incorrectly treating that loaded edge as the dataset end.

Resolution

Current navigation keeps the down arrow visible when total count is unknown and more pages remain:

currentPage < pageCount - 1
    || (!paginationTotalKnown && hasMorePages)

Covered by test_unbounded_pages_keep_down_arrow_at_loaded_edge(). Implemented in 0b3eee4 / #377.

[wizzomafizzo]

Resolved in current source by #377, with regression coverage for unknown-total pagination at the loaded edge.