[Feature] Support vector and full-text search in File Index

Start with the existing File Index write path, container format, readers, and the work from #173. Trace how read-context requests reach FileIndexReader and how selected row positions reach the data-file reader. Done means vector and full-text indexes round-trip through write/read paths, support the listed edge cases, and preserve per-file TopK score alignment.

Search before asking

• I searched in the issues and found nothing similar.

Motivation

Paimon C++ has File Index infrastructure for per-data-file indexes, but does not yet provide an end-to-end File Index search path for vector similarity or full-text queries. Applications should be able to build these indexes with their data files and use them to select matching file-local rows without requiring a Global Index.

Solution

Add Vector Search and Full Text Search support to File Index:

• Build and serialize vector and full-text index artifacts through the existing File Index write path and container format, and open them through File Index readers.
• Add file-local search visits to FileIndexReader and route search requests from the read context to the appropriate index reader.
• Apply returned file-local row positions to the data-file reader. Vector Search should return a TopK result independently for each data file and keep scores aligned with selected rows; expose scores through an explicitly projected _INDEX_SCORE column.
• Support Full Text Search row selection. Score output for Full Text Search can be handled separately.
• Add focused tests for write/read round trips, null or empty indexes, multiple data files and batches, score alignment, and columns with more than one File Index type.

The initial implementation can target append-only/raw-file reads. Predicate combination, deletion-vector and row-range pre-filtering, Merge-on-Read/Data Evolution, and cross-file global TopK should be handled in follow-up work so that filtering does not silently change TopK semantics.

Anything else?

This builds on the File Index writing work in #173; it is specifically about vector/full-text index implementations and their search read path.

Are you willing to submit a PR?

• I'm willing to submit a PR!