fix(fts): enforce fuzzy max_expansions globally across index partitions

[BubbleCal]

Follow-up to the review discussion on #7601: fuzzy expansion previously ran
inside every partition with a per-partition max_expansions cap, and the
per-partition picks were unioned afterwards. The effective cap was
num_partitions × max_expansions, so the same query could match more terms —
and return different results — purely because of how the corpus happened to be
partitioned (e.g. after a tail-heavy build splits its leftovers).

What

• Expansion now runs once per query, in InvertedIndex::bm25_search,
  under the query-wide max_expansions budget. For each query token the
  per-partition FST candidates merge into one lexicographically ordered set
  and the remaining budget takes a prefix of it. Each partition is only asked
  for its remaining lexicographically smallest candidates, which is lossless
  for that selection: any term among the merged lex-smallest remaining is
  also among its own partition's smallest remaining.
• Partitions receive the final token list and no longer expand
  (load_posting_lists drops its expand_fuzzy call); params.fuzziness
  still drives the grouped fuzzy dedup/scoring semantics.
• The BM25 scorer reuses the same expansion (bm25_scorer_for_final_tokens),
  so scorer terms and searched terms stay in lockstep and a query pays for one
  expansion instead of one for the scorer plus one per partition.
• Fuzzy AND/phrase keep their contract — every original token position
  must retain at least one expansion — via an index-level check (previously
  implicit in each partition's own expansion).
• InvertedPartition::expand_fuzzy stays for API compatibility,
  reimplemented on the shared per-token candidate collector; single-partition
  behavior is unchanged (same FST lexicographic order, same budget fill).

Semantics

Single-partition indexes behave exactly as before. Multi-partition indexes
previously over-expanded in proportion to their partition count; they now
honor the documented cap, and the expansion (hence the result set) is a pure
function of the segment's vocabulary, independent of partition shape.

Tests

• test_fuzzy_expansion_cap_is_global_across_partitions: two partitions with
  disjoint variants, binding cap → exactly the 3 lexicographically smallest
  terms across both (fails on main, which returns 4).
• test_fuzzy_results_independent_of_partition_shape: the same four docs
  built as one partition and as two return identical (row_id, score) sets
  under a binding cap (fails on main).
• Existing fuzzy suite (grouped AND, position grouping, whole-query cap)
  unchanged and passing. The non-binding-cap regression test over the real
  tail-split builder path lives in fix(fts): split tail-partition merge by the worker memory budget #7601.

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Codecov Report

❌ Patch coverage is 92.81768% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/inverted/index.rs	92.81%	10 Missing and 3 partials ⚠️

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