Add dump subcommand and introduce reusable group filtering arguments

Introduces a new `dump` CLI command to export index k-mers as a CSV table. Adds a reusable `GroupFilterArgs` struct for ingroup/outgroup metadata-predicate quorum filtering with configurable count and fraction thresholds. Refactors the filter command to use explicit flags and a `GenomeSelector`-based pipeline, improving error handling and predicate application order. Exposes `IndexDump` from the `obikdump` crate and updates local dependencies accordingly.

src/obikdump/src/lib.rs

@@ -6,3 +6,5 @@
 //! reverse), same pattern as `obikindexer`/`obikquery`.
 
 mod dump;
+
+pub use dump::IndexDump;