fix(doctest): use ndarray::hpc:: not crate::hpc:: in stream doctests (codex P2)

Codex P2 flagged that doctest examples compile as an external test
crate, so `crate::hpc::...` doesn't resolve — `cargo test --doc`
would fail with "could not find hpc in the crate root". W-F5's
InferenceStream got this right; W-F4 (QualiaStream) and W-F6
(SplatFieldStream) used `crate::hpc::...` and would have broken
doctest CI.

Two doctest import paths corrected to use the external crate
name `ndarray::hpc::...` (matching the W-F5 pattern).

Author: claude

src/hpc/stream/qualia.rs

@@ -26,7 +26,7 @@ pub struct QualiaI4Row(pub u64);
 /// # Example
 ///
 /// ```
-/// use crate::hpc::stream::qualia::{QualiaI4Row, QualiaStream};
+/// use ndarray::hpc::stream::qualia::{QualiaI4Row, QualiaStream};
 ///
 /// let rows = vec![QualiaI4Row(1), QualiaI4Row(2), QualiaI4Row(3)];
 /// let mut stream = QualiaStream::new(&rows);

src/hpc/stream/splat_field.rs

@@ -35,7 +35,7 @@ pub struct SplatField {
 /// # Example
 ///
 /// ```
-/// use crate::hpc::stream::splat_field::{SplatField, SplatFieldStream};
+/// use ndarray::hpc::stream::splat_field::{SplatField, SplatFieldStream};
 ///
 /// let rows = vec![
 ///     SplatField { mean: 0, variance: 1.0, energy: 0.5, generation: 1 },