Add unitig streaming iterators and estimate CLI subcommand

Introduce `iter_unitigs` methods across the index cache, content layer, MphfLayer, and typed layer to stream whole reconstructed sequences directly from underlying storage without decomposing into k-mers. Add a corresponding low-level streaming iterator in obiskio for thread-safe, lazy reads from memory-mapped files. Include a new CLI subcommand to compute and display approximate false-positive rates based on provided indexing parameters.

src/obikindex/src/layer/content_layer.rs

@@ -311,4 +311,16 @@ impl KmerLayer {
             }
         }
     }
+
+    /// Iterate over the layer's unitigs (whole sequences, not decomposed
+    /// into k-mers) — the "layer level" step of the layer/partition/index
+    /// chain of unitig iterators; see `KmerPartition::iter_unitigs`/
+    /// `KmerIndex::iter_unitigs`.
+    pub fn iter_unitigs(&self) -> crate::layer::mphf_layer::UnitigIter {
+        match self {
+            KmerLayer::Count { layer, .. } => layer.iter_unitigs(),
+            KmerLayer::Presence { layer, .. } => layer.iter_unitigs(),
+            KmerLayer::Empty { .. } => panic!("Layer::iter_unitigs() called on an Empty layer"),
+        }
+    }
 }

src/obikindex/src/layer/mod.rs

@@ -9,5 +9,5 @@ pub(crate) mod utils;
 pub use crate::index::error::{OKIError, OKIResult};
 pub use content_layer::KmerLayer;
 pub use meta::IndexMode;
-pub use mphf_layer::{EvidenceKind, KmerBatchIter, KmerIter, MphfLayer, MphfOnly};
+pub use mphf_layer::{EvidenceKind, KmerBatchIter, KmerIter, MphfLayer, MphfOnly, UnitigIter};
 pub use typed_layer::{HasLayerContent, HasStorageKind, Hit, LayerContent, LayerData, TypedLayer};

src/obikindex/src/layer/mphf_layer.rs

@@ -341,6 +341,16 @@ impl MphfLayer {
             (base, batch)
         })
     }
+
+    /// Iterate over the layer's unitigs (whole reconstructed sequences, not
+    /// decomposed into k-mers), in the physical order of `unitigs.bin`. Same
+    /// `Send + 'static` shape as [`iter_kmers`](Self::iter_kmers), via
+    /// `UnitigFileReader::iter_unitigs_owned`.
+    pub fn iter_unitigs(&self) -> UnitigIter {
+        UnitigIter {
+            inner: Box::new(self.unitigs.iter_unitigs_owned()),
+        }
+    }
 }
 
 // ── Iterator types ────────────────────────────────────────────────────────────
@@ -365,6 +375,24 @@ impl Iterator for KmerIter {
     }
 }
 
+/// Iterator over the unitigs stored in a layer, whole (not decomposed into
+/// k-mers). Produced by [`MphfLayer::iter_unitigs`]. Same ownership shape as
+/// [`KmerIter`] — owns an `Arc<UnitigFileReader>` clone, `Send + 'static`,
+/// streamed from disk.
+pub struct UnitigIter {
+    inner: Box<dyn Iterator<Item = (usize, obikseq::Unitig)> + Send>,
+}
+
+impl Iterator for UnitigIter {
+    type Item = obikseq::Unitig;
+
+    /// Return the next unitig in iteration order (its `chunk_id` dropped —
+    /// use [`Iterator::enumerate`] if the index within the layer is needed).
+    fn next(&mut self) -> Option<Self::Item> {
+        self.inner.next().map(|(_, unitig)| unitig)
+    }
+}
+
 /// Iterator over batches of canonical kmers stored in a layer.
 ///
 /// Produced by [`MphfLayer::iter_kmers_batch`].  Each call to [`next`](Self::next)

src/obikindex/src/layer/typed_layer.rs

@@ -223,6 +223,12 @@ impl<D: LayerData> TypedLayer<D> {
         self.mphf.enumerate_kmers_batch(n)
     }
 
+    /// Iterate over the layer's unitigs (whole sequences, not decomposed
+    /// into k-mers).
+    pub fn iter_unitigs(&self) -> crate::layer::mphf_layer::UnitigIter {
+        self.mphf.iter_unitigs()
+    }
+
     pub fn unitig_writer(out_dir: &Path) -> OKIResult<UnitigFileWriter> {
         MphfLayer::unitig_writer(out_dir)
     }