bug: FTS corpus statistics depend on the query filter and index coverage

BM25 scores change when neither the data nor the query changes, because the corpus the scores are measured against is derived partly from the rows a particular query happens to read.

Two triggers with the same root cause.

1. The user filter moves the corpus

The flat side of a partially indexed plan is read through filtered_read(filter_plan, ...), and initialize_scorer folds counted_input.num_rows() from that already-filtered scan into num_docs. Filtered-out tail rows are therefore missing from the corpus, while the index statistics still count filtered-out indexed rows.

2. Index coverage moves the corpus

A fully covered fragment selection is scored against index-only statistics, even when unindexed fragments exist outside the selection. Once optimize_indices() folds those fragments in, the same rows score differently.

Reproduce

Two indexed rows (title = alpha, beta), ten alpha rows appended unindexed, limit=1. Run the query, then optimize_indices(), then run it again. No searchable data changed:

Both hold for single-column match too, where the returned row changes as well, not just its score: [(0, 0.6931472)] → [(1, 2.1594841)].

Scope

Affects single-column match and combined_fields alike — plan_match_query / plan_flat_match_query / initialize_scorer and the FlatFieldStats / SharedFtsScorer path have the same shape.

The overlay-triggered form of this invariant is already fixed for combined_fields in #7905 (FlatScanFilter::AtEmission, pinned by test_fts_combined_fields_overlay_preserves_prefilter_corpus), since a same-value overlay must not move a ranking. The filter- and coverage-triggered forms remain, for both query types.

Fix

Build one corpus that is independent of the filter and of index coverage, before either child runs, and apply the user filter only to emitted candidates. Candidate selection can stay fragment-scoped; only corpus construction has to be global.

Found in review of #7905:
https://github.com/lance-format/lance/pull/7905#discussion_r3955108697 and
https://github.com/lance-format/lance/pull/7905#discussion_r3955108708

Start at plan_match_query, plan_flat_match_query, and initialize_scorer, then trace the FlatFieldStats and SharedFtsScorer paths for both match and combined_fields. Reproduce the filter and index-coverage cases described in the issue, and compare them with test_fts_combined_fields_overlay_preserves_prefilter_corpus from #7905. Done means corpus statistics and returned scores or rows remain stable when applying a user filter or optimizing index coverage.