fix(rank-fusion): show previous page on multi-page semantic-only results

[marevol]

Problem

When rank fusion runs and the primary (BM25 / default) searcher returns few or zero hits while a secondary searcher (e.g. a semantic/multimodal searcher) returns many, pagination breaks on multi-page result sets:

• prev_page is false on every page past the first, so themes that key their pager on prev_page/next_page hide the whole control and the user cannot navigate back.
• page_count inflates by one, one page past the actual end.

Example: a query that matches 37 documents only via the secondary searcher (4 pages of 10) reports prev_page=false on pages 2–4 and page_count=5 past the end. A normal keyword query paginates correctly, because BM25 contributes to the main searcher's hit set.

Root cause

QueryResponseList's offset field does double duty:

• The rank-fusion merge inflates the effective record count by offset when the primary searcher contributes few/zero hits (allRecordCount = mainCount + offset).
• calculatePageInfo() also reused that same offset-adjusted value (startWithOffset = start - offset) as a navigation origin-shift for the pagination flags.

When the primary searcher returns ~0 hits, offset inflates to roughly the full result count, so startWithOffset clamps to 0 on every page. That makes existPrevPage = startWithOffset > 0 false on all pages, and a spurious next page one page past the end is then collapsed into an inflated allPageCount.

Fix

Decouple the navigation flags from the count-inflation role inside calculatePageInfo():

• existPrevPage now also considers currentPageNumber — a previous page exists whenever we are past page 1.
• existNextPage is computed from the raw start, keeping it consistent with allPageCount (which is derived from allRecordCount), instead of the offset-deflated startWithOffset.

No other pagination logic changes. For offset == 0 (all non-fusion / single-searcher paths) the behavior is provably identical to before, and normal deep-pagination behavior is preserved.

Tests

Added QueryResponseListTest#test_calculatePageInfo_rankFusionZeroMainMultiPage covering the zero-primary multi-page case: previous page present on pages 2–4, no spurious next page, page_count stays 4, and no previous page on page 1.

QueryResponseListTest (38 tests) and RankFusionProcessorTest (11 tests) pass with no regressions.