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/obiskio/src/unitig_index/reader.rs

@@ -208,6 +208,31 @@ impl UnitigFileReader {
         self.iter_chunks_sequential()
     }
 
+    /// Same streamed sequence as [`iter_unitigs`](Self::iter_unitigs), but
+    /// owning a clone of `self` instead of borrowing it — `Send + 'static`,
+    /// so it can be handed to a threaded consumer without first collecting
+    /// the layer's unitigs into memory. Same shape as
+    /// [`iter_indexed_canonical_kmers_owned`](Self::iter_indexed_canonical_kmers_owned),
+    /// stopping short of decomposing into k-mers.
+    pub fn iter_unitigs_owned(
+        self: &Arc<Self>,
+    ) -> impl Iterator<Item = (usize, Unitig)> + Send + 'static {
+        let this = Arc::clone(self);
+        let k = this.k;
+        let n = this.n_unitigs;
+        let mut offset = 0usize;
+        (0..n).map(move |chunk_id| {
+            let mmap = &*this.mmap;
+            let seql = mmap[offset] as usize + k;
+            let byte_len = (seql + 3) / 4;
+            let bytes = mmap[offset + 1..offset + 1 + byte_len]
+                .to_vec()
+                .into_boxed_slice();
+            offset += 1 + byte_len;
+            (chunk_id, Unitig::new((seql % 4) as u8, bytes))
+        })
+    }
+
     pub fn iter_kmers(&self) -> impl Iterator<Item = Kmer> + '_ {
         self.iter_chunks_sequential()
             .flat_map(|(_, u)| u.into_kmers())