[core] Unify single-column global index writer

[JingsongLi]

Summary

This PR merges the previous singleton and parallel single-column global index writer APIs into one GlobalIndexSingleColumnWriter interface. Single-column index writers now receive the caller-provided shard-relative row id through write(@Nullable Object key, long relativeRowId).

Changes

• Replace GlobalIndexSingletonWriter and GlobalIndexParallelWriter with GlobalIndexSingleColumnWriter.
• Update BTree, Vector, Lumina, and Tantivy global index writers to implement the unified single-column writer API.
• Pass explicit shard-relative row ids from BTree, Flink, and Spark index build paths.
• Keep vector/full-text row counts as logical row counts while persisting caller-provided row ids for non-null indexed entries.
• Update test helper index formats and affected tests to read/write explicit relative row ids.

Testing

• mvn -pl paimon-vector/paimon-vector-index -am -Pfast-build -DskipTests test-compile
• mvn -pl paimon-lumina -am -Pfast-build -DskipTests test-compile
• mvn -pl paimon-tantivy/paimon-tantivy-index -am -Pfast-build -DskipTests test-compile
• mvn -pl paimon-flink/paimon-flink-common -am -Pfast-build -DskipTests test-compile
• mvn -pl paimon-spark/paimon-spark-common -am -Pfast-build -DskipTests compile
• git diff --check

[leaves12138]

Thanks for the update. I re-reviewed the latest revision and the single-column writer API now looks consistent across BTree, vector, Lumina, Tantivy, Flink and Spark build paths.

The important semantic change is also correct: vector/full-text writers now persist the caller-provided relative row id instead of relying on an implicit dense sequence, while rowCount still represents logical rows processed for index metadata. This matches sparse/sharded row ranges and the added multiple-index-file tests cover the regression case.

I also ran the focused checks locally:

• mvn -pl paimon-core -am -Pfast-build -DfailIfNoTests=false -Dtest='VectorSearchBuilderTest#testVectorSearchWithMultipleIndexFiles,FullTextSearchBuilderTest#testFullTextSearchWithMultipleIndexFiles' test
• mvn -pl paimon-lumina -am -Pfast-build -DfailIfNoTests=false -Dtest='LuminaVectorGlobalIndexWriterTest' test

Both passed. +1 from my side.