feat: add --head and --presence-threshold to dump and distance

Introduces `--head N` to the `dump` command for early iteration termination and `--presence-threshold N` to the `distance` command for Jaccard filtering on count indexes. Updates filter defaults to adapt based on explicit ingroup/outgroup declarations. Fixes a Rust type mismatch in the unitig closure and updates partition iteration callbacks to return `bool` for proper early termination support. Documentation is updated accordingly.

docmd/index.md

@@ -11,9 +11,9 @@
 | `merge`     | Merge multiple built indexes into one |
 | `rebuild`   | Filter and compact an existing index into a new single-layer index; supports ingroup/outgroup predicates on genome metadata |
 | `query`     | Query an index with sequences and annotate matches |
-| `dump`      | Dump all indexed k-mers as CSV (kmer + per-genome counts or presence); supports the same ingroup/outgroup filtering as `rebuild` |
+| `dump`      | Dump all indexed k-mers as CSV (kmer + per-genome counts or presence); supports the same ingroup/outgroup filtering as `rebuild`; `--head N` limits output to the first N k-mers |
 | `annotate`  | Add or update genome metadata from a CSV file; or dump metadata as CSV |
-| `distance`  | Compute pairwise distance matrix between genomes; optionally build NJ/UPGMA trees |
+| `distance`  | Compute pairwise distance matrix between genomes; optionally build NJ/UPGMA trees; `--presence-threshold N` sets the minimum count to consider a k-mer present when computing Jaccard on count indexes (default 1) |
 | `unitig`    | Build a global de Bruijn graph across all partitions and enumerate its unitigs as FASTA; supports the same ingroup/outgroup filtering as `rebuild` |
 | `estimate`  | Estimate approximate-index parameters (z, evidence bits, FP rates) before indexing |
 | `reindex`   | Convert an index's evidence in-place: exact ↔ approx |