style(simd_avx2): apply nightly rustfmt to U8x32 additions (PR #144 fmt fix)

The format/nightly CI job on PR #144 flagged two sites in the U8x32
additions:

1. `nibble_popcount_lut` — 32-byte literal split into two 16-element
   rows for readability. Nightly rustfmt's chains_overflow_last_block
   + width budget collapse it to one line. Restored.

2. `permute_bytes_reverse` test — a 3-method chain was on one line.
   Nightly rustfmt wants each `.method()` on its own line under
   `chain_width = 60`. Restored.

No semantic change. `cargo +nightly fmt --all --check` clean after.

Author: claude

src/simd_avx2.rs

@@ -2004,8 +2004,7 @@ impl U8x32 {
     pub fn nibble_popcount_lut() -> Self {
         // Index i ∈ [0,15] → number of set bits in i.
         Self::from_array([
-            0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4,
-            0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4,
+            0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4, 0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4,
         ])
     }
 }
@@ -2209,7 +2208,9 @@ mod u8x32_tests {
     fn permute_bytes_reverse() {
         let src: [u8; 32] = core::array::from_fn(|i| i as u8);
         let idx: [u8; 32] = core::array::from_fn(|i| (31 - i) as u8);
-        let out = U8x32::from_array(src).permute_bytes(U8x32::from_array(idx)).to_array();
+        let out = U8x32::from_array(src)
+            .permute_bytes(U8x32::from_array(idx))
+            .to_array();
         for i in 0..32 {
             assert_eq!(out[i], src[31 - i]);
         }