fix(pillar/koestenberger): import Spd3 via linalg (consistent with pillar deps)

The leftover diff from the earlier B3 import-path patch wasn't committed
before the Cargo.toml splat3d-feature fix. Now that pillar = [linalg, splat3d]
both paths resolve to the same Spd3 type (linalg re-exports splat3d::Spd3
when splat3d feature is on), so the linalg path is the canonical one.

Author: claude

src/hpc/pillar/koestenberger.rs

@@ -31,7 +31,8 @@
 
 use crate::hpc::linalg::eig_sym::eig_sym_3;
 use crate::hpc::pillar::prove_runner::{PillarReport, SplitMix64, random_contractive_spd3};
-use crate::hpc::splat3d::spd3::{sandwich, Spd3};
+use crate::hpc::linalg::Spd3;
+use crate::hpc::splat3d::spd3::sandwich;
 
 // ── Probe constants ────────────────────────────────────────────────────────────