chore: Fix clippy warning by feature-gating MaskValues serde helpers

The `from_buffer` and `from_buffer_with_counts` methods are only used
by the serde implementation, so they should be feature-gated to avoid
dead code warnings when the serde feature is disabled.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: blaginin

vortex-mask/src/lib.rs

@@ -127,11 +127,13 @@ pub struct MaskValues {
 }
 
 impl MaskValues {
-    fn from_buffer(buffer: BitBuffer) -> Self {
+    #[cfg(feature = "serde")]
+    pub(crate) fn from_buffer(buffer: BitBuffer) -> Self {
         let count = buffer.true_count();
         Self::from_buffer_with_counts(buffer, count)
     }
 
+    #[cfg(feature = "serde")]
     fn from_buffer_with_counts(buffer: BitBuffer, true_count: usize) -> Self {
         let len = buffer.len();