[Feature](lance) Add radius and full-text search modes

## Parent

Tracked by #66340. ANN and Flat vector search were introduced by #65730.

## Scope

- [ ] Add Radius/Range Search with lower and upper distance bounds.
- [ ] Add standalone full-text search with BM25.
- [ ] Define result schemas, filtering order, scoring semantics, limits, and EXPLAIN output.
- [ ] Keep Hybrid Search out of scope unless it is designed separately.

## Completion criteria

- [ ] FE analysis and BE/lance-c execution paths are implemented.
- [ ] Unit and regression tests cover bounds, BM25 scoring, filters, and invalid parameters.
- [ ] User documentation describes syntax, score or distance semantics, and limitations.
- [ ] Representative indexed and non-indexed datasets are verified.

Start with parent issue #66340 and the ANN/Flat vector search work from #65730, then trace the FE analysis and BE/lance-c execution paths named here. The work is complete when radius bounds, BM25 scoring, filters, invalid parameters, result schemas, and EXPLAIN behavior have tests, representative indexed and non-indexed datasets are verified, and user documentation covers syntax and limitations.

Written by the indexing model from the issue text.