fix(doctests): fix 3 doctest failures breaking CI

1. crystal_encoder::distill — remove unused import `Role`
2. nibble::nibble_propagate_bfs — remove unused import `nibble_unpack`
3. udf_kernels::udf_sigma_classify — swap band/threshold args in
   doctest assertions (distance=100 with threshold=1000 → Foveal,
   not distance=1000 with threshold=100 → Reject)

All 3 fail under CI's RUSTFLAGS="-D warnings". Now: 335 doctests pass,
0 fail, 7 ignored.

Co-authored-by: AdaWorldAPI <AdaWorldAPI@users.noreply.github.com>

Author: cursoragent

src/hpc/crystal_encoder.rs

@@ -354,7 +354,7 @@ fn distillation_loss(student: &mut Node, teacher: &mut Node, lambda: f32) -> f32
 /// # Example
 ///
 /// ```
-/// use ndarray::hpc::crystal_encoder::{CrystalEncoder, Role, distill};
+/// use ndarray::hpc::crystal_encoder::{CrystalEncoder, distill};
 /// use ndarray::hpc::node::Node;
 ///
 /// let teacher = Node::random(42);

src/hpc/nibble.rs

@@ -301,7 +301,7 @@ pub(crate) unsafe fn nibble_above_threshold_avx2(packed: &[u8], threshold: u8) -
 /// # Examples
 ///
 /// ```
-/// use ndarray::hpc::nibble::{nibble_pack, nibble_propagate_bfs, nibble_unpack};
+/// use ndarray::hpc::nibble::{nibble_pack, nibble_propagate_bfs};
 /// let mut packed = nibble_pack(&[5, 3, 10, 1, 0, 15, 2, 7]);
 /// let frontier = nibble_propagate_bfs(&mut packed, 3);
 /// // After subtracting 3: [2, 0, 7, 0, 0, 12, 0, 4]

src/hpc/udf_kernels.rs

@@ -188,8 +188,8 @@ pub fn udf_nars_revision(freq1: u16, conf1: u16, freq2: u16, conf2: u16) -> (u16
 /// ```
 /// use ndarray::hpc::udf_kernels::udf_sigma_classify;
 ///
-/// assert_eq!(udf_sigma_classify(1000, 100), "exact");
-/// assert_eq!(udf_sigma_classify(1000, 1500), "noise");
+/// assert_eq!(udf_sigma_classify(100, 1000), "exact");
+/// assert_eq!(udf_sigma_classify(1500, 1000), "noise");
 /// ```
 pub fn udf_sigma_classify(band: u32, threshold: u64) -> &'static str {
     use super::cascade::Band;