[core] Support TopN pushdown for BTree global indexes

[discivigour]

What changed

• add a TopN result type that keeps sortable keys and merges candidates globally across BTree readers
• support forward and reverse BTree iteration for ascending and descending TopN, including null ordering
• prune dominated BTree index files before lookup while retaining the complete file set for coverage calculation
• group candidate row IDs by key and pass the resulting row ranges into data-evolution scans
• add unit and table-level coverage for result merging, iterator direction, file selection, cross-file candidates, diagnostics, and fallback behavior

Why

BTree global indexes could evaluate predicates, but TopN planning could not use the index ordering to reduce index and data reads. Evaluating each file independently is insufficient because the final candidates must be compared globally, and coverage handling must not treat intentionally pruned index files as unindexed data.

This change computes a conservative global candidate set for supported single-column TopN queries. The execution engine still retains the final sort and limit, so the pushdown remains a pruning optimization rather than the source of final result ordering.

Impact

Data-evolution tables with a compatible BTree global index can reduce index-file opens, index-entry scans, and downstream row reads for ascending or descending TopN queries.

Validation

mvn -pl paimon-core -am \
  -Dmaven.repo.local=/opt/homebrew/opt/maven/repository \
  -DwildcardSuites=none \
  -DfailIfNoTests=false \
  -Dtest=GlobalIndexEvaluatorTest,TopNGlobalIndexResultTest,BTreeIndexReaderTest,LazyFilteredBTreeIndexReaderTest,BlockIteratorTest,BTreeTopNIndexFileSelectorTest,BtreeGlobalIndexTableTest \
  test

• paimon-common: 229 tests passed
• paimon-core: 27 tests passed
• Checkstyle, Spotless, and Maven Enforcer passed
• git diff --check passed

Submitted by mistake.

[JingsongLi]

Can you add tests for Spark SQL?

[JingsongLi]

Please disable global-index TopN for incremental scan domains.