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/implementation/rebuild_filter.md

@@ -96,8 +96,16 @@ For each genome:
 | `--max-total-count N` | all genomes | sum of per-genome counts ≤ N (`rebuild` only) |
 | `--presence-threshold N` | all | per-genome count > N to be considered "present" (default 0) |
 
-Defaults: mins = 0 (no lower bound), max counts = group size, max fracs = 1.0
-(no upper bound). A filter with all defaults is a no-op.
+**Conditional defaults** — the default for `--min-frac` and `--max-outgroup-count` depends on whether the corresponding group was explicitly declared:
+
+| Situation | `--min-frac` default | `--max-outgroup-count` default |
+|-----------|----------------------|-------------------------------|
+| Neither `--ingroup` nor `--outgroup` | 0.0 (no-op) | no constraint (no-op) |
+| `--ingroup` only | **1.0** — all ingroup genomes must carry the k-mer | no constraint |
+| `--outgroup` only | 0.0 | **0** — no outgroup genome may carry the k-mer |
+| Both declared | **1.0** | **0** |
+
+Explicit flags always override these defaults. All other bounds (`--min-count`, `--max-count`, `--max-frac`, `--min-outgroup-*`) default to 0 / group size / 1.0 regardless of whether groups are declared.
 
 Fractions are computed over the size of the classified group, not over total
 genome count. An empty group (no genome classified as ingroup/outgroup) never
@@ -169,6 +177,31 @@ obikmer unitig myindex \
   --max-outgroup-count 0
 ```
 
+## Command-specific options
+
+### `dump --head N`
+
+Stops output after the first N k-mers that pass all active filters.
+Iteration terminates immediately — subsequent partitions and layers are not scanned.
+Useful for quick inspection of large indexes without loading the entire dataset.
+
+```sh
+obikmer dump myindex --head 100
+obikmer dump myindex --head 20 --ingroup "species=Betula_nana" --min-count 1
+```
+
+### `distance --presence-threshold N`
+
+When computing Jaccard distance on a **count index**, a k-mer is considered present in a genome if its count is ≥ N (default 1).
+This option is independent of the `--presence-threshold` used in `rebuild`/`query` filtering.
+
+```sh
+# Jaccard treating kmers with count ≥ 2 as present
+obikmer distance myindex --metric jaccard --presence-threshold 2
+```
+
+This parameter has no effect on presence/absence indexes (where values are already 0/1) or on metrics other than Jaccard.
+
 ## Implementation
 
 - **`obikpartitionner::filter::GroupQuorumFilter`** — implements `KmerFilter`