feat(java): expose global BM25 statistics for FTS queries

[zhangstar333]

• Collect query-bound BM25 statistics across all committed segments of a single logical FTS index.
• Preserve the prepared vocabulary and original token positions for each scoring leaf, including fuzzy expansions.
• Expose the statistics through the Java/JNI API as an opaque protobuf payload.
• Reject queries whose scoring leaves span multiple FTS corpora.

This is the implementation for #8937 and depends on #8938.

It supports the distributed FTS use case described in apache/doris#67435.

[github-actions]

Important

This PR touches the Lance format specification.

Substantive changes to the format specification — the .proto definitions
and the spec docs under docs/src/format/ — require a PMC vote before merge.
Minor edits such as typo fixes, wording, or formatting are excluded; use your
judgment.

If this is a meaningful format change:

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  Format specification modifications need 3 binding +1 votes (excluding the
  proposer), held on GitHub Discussions, with a minimum voting period of 1 week.
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  merged until the vote is linked.

[github-actions]

Important

Format specification vote

This PR modifies the Lance format specification, so it requires 3 binding +1 votes from PMC members (excluding the proposer) and a minimum 72-hour voting period, weekends excluded, before it can merge. Vote by approving this PR (+1) or requesting changes (−1, a veto). See the voting process.

Status: ❌ Blocked — 0 of 3 required approvals

Approvals (this commit)	none (0/3)
Vetoes	none
Voting period	elapsed — ended Fri 2026-09-04 03:56 UTC (Thu 20:56 PDT)

_{Updated automatically by the format-spec vote gate, which re-checks every 15 minutes — just voted? Re-check now (press Run workflow; leave the input blank to re-check every open format PR). A PMC member may apply the format-waived label to waive the vote for a trivial edit (typo, wording, formatting).}

[lance-gatekeeper]

⚠️ Gate recommendation: approve with a non-blocking risk.

The contract change in #8919 now scopes separate format proposals to persisted Lance formats, so the prior format-process finding no longer applies to this execution-only payload. This revision also carries deterministic prepared query leaves and token positions, closing the earlier fuzzy-replay risk.

The remaining integration risk is that the downstream lance-c consumer tracked by #8937 is outside this revision, so snapshot/query mismatch rejection and distributed Top-K equivalence are not yet exercised through the wire. No further producer change is requested here.