refactor: add rolling buffer methods and document label constraints

Added `is_empty()`, `clear()`, and `iter()` methods to the rolling statistics buffer to enable standard traversal and state reset operations. Documented genome label constraints, specifying forbidden characters, empty label rejection, space quoting requirements, and auto-derived label bypass rules. Additionally, updated doc comments and added `#[allow(dead_code)]` attributes for `kmer_offset` and `n_kmers` fields to suppress compiler warnings while reserving them for future `--detail` coverage vector logic.

src/obiskbuilder/src/rolling_stat.rs

@@ -24,10 +24,6 @@ impl<T: Copy + Default, const N: usize> Ring<T, N> {
         }
     }
     #[inline]
-    fn is_empty(&self) -> bool {
-        self.len == 0
-    }
-    #[inline]
     fn clear(&mut self) {
         self.len = 0;
         self.head = 0;
@@ -67,10 +63,6 @@ impl<T: Copy + Default, const N: usize> Ring<T, N> {
         }
     }
 
-    /// Iterate over elements front-to-back (copies, since T: Copy).
-    fn iter(&self) -> impl Iterator<Item = T> + '_ {
-        (0..self.len).map(move |i| self.buf[(self.head + i) % N])
-    }
 }
 
 // ── MmerItem ──────────────────────────────────────────────────────────────────