Fix minor noise projection in blockwise singlepass via final fixed-V LS refit

[Lindsey-cyber]

Summary

This PR fixes a consistency issue in blockwise_decomposition_singlepass.

After the final temporal basis V is computed (and optionally denoised), we now recompute the spatial basis U using a least-squares fit on the original normalized block.

This removes the extra spatial-projection path and ensures that the returned U is the optimal fit for the final V.

What Changed

1. Keep the existing pipeline for estimating the final temporal basis V.

2. Before returning, recompute U by solving:

   U = argmin ||X - U V||_F^2
   

   using a QR-based solve.

3. No change to behavior for empty or rank-0 edge cases.

Note: No algorithmic change to temporal basis estimation itself. By refitting U with V fixed,U is now consistent with the actual basis used for reconstruction. Minor projection artifacts are removed.