feat: add Vortex columnar file format support as optional feature (#260)

* feat: add Vortex columnar file format support as optional feature

Add read/write support for the Vortex file format behind a `vortex` feature flag.
The writer uses a background task with kanal channel for streaming writes, and the
reader supports predicate pushdown and row selection. Also introduces a configurable
`file.format` option (default "parquet") to replace hardcoded file extensions across
all writer paths.

* fix: remove eprintln in library code and fix stale comment

Remove eprintln! warning from VortexFormatWriter Drop impl, and update
stale "Open parquet writer" comment in kv_file_writer.rs to be format-agnostic.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix comment

* add vortex to ci

* Fix comment

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>