fts: fix stopword maintenance and transactional statistics

[jkumz]

Summary

This PR fixes three FTS consistency bugs:

• Index maintenance skipped configured stopword filtering when stemming was disabled.
• A stopword-only document stopped the remainder of an insertion batch from being indexed.
• FTS document statistics were retained after insert or delete transactions rolled back.

Changes

Apply consistent stopword filtering

Apply configured stopword filtering to conjunctive index-maintenance paths when stemmer := 'none'.

This keeps initial index construction, runtime insertion, and deletion consistent while preserving the existing no-stemming behavior for non-conjunctive query paths.

Preserve batch processing

Skip documents that produce no indexed terms without terminating the remaining batch.

The insertion and deletion paths now:

• Count only documents that produce indexed terms.
• Calculate average document length from indexed documents only.
• Resolve indexed property vectors using their logical batch positions and selection vectors.
• Continue processing documents after a stopword-only entry.

Make FTS statistics transactional

Accumulate document-count, total-document-length, and checkpoint-watermark changes per transaction.

• Commit callbacks apply pending deltas to the committed aggregate.
• Rollback callbacks discard pending deltas.
• Queries read committed statistics plus their own transaction’s pending changes.
• Checkpoint serialization snapshots committed statistics under synchronization.

Testing

• Rebuilt lbug_fts_extension.
• Passed focused default and custom stopword tests.
• Passed COPY and UNWIND stopword-only batch tests.
• Passed live and close/reopen deletion tests.
• Passed insert and delete rollback metadata tests.
• Passed focused update and recovery regressions.
• Passed the FTS extension CTest targets: 2/2.
• Passed clang-format --dry-run --Werror.
• Passed git diff --check.

Fixes #39 #40 #41