Fix vector corruption in Panorama merge_from (#5607)

[Ritabanm]

Fixes #5607

Summary

When merging an index backed by ArrayInvertedListsPanorama using merge_from(), vectors transferred from the source index were corrupted because InvertedLists::merge_from() called ScopedCodes(oivf, i).get() (which returns Panorama's internal level-major layout) and passed it to add_entries() (which expects flat row-major vectors).

Changes

• faiss/invlists/InvertedLists.cpp & .h:
  • Made InvertedLists::merge_from() virtual.
  • Reconstruct codes into flat row-major float vectors using pano.reconstruct() before calling add_entries() when merging from an ArrayInvertedListsPanorama.
  • Reconstruct vectors in InvertedLists::copy_subset_to() for SUBSET_TYPE_INVLIST when source is ArrayInvertedListsPanorama.
• faiss/IndexIVFFlatPanorama.cpp & .h: Added check_compatible_for_merge() override to enforce n_levels and batch_size consistency.
• faiss/IVFlib.cpp: Handled Panorama vector reconstruction in get_invlist_range().
• tests/test_merge_index.py: Added unit test test_merge_IVFFlatPanorama validating vector reconstruction fidelity and exact equivalence of search results with vanilla IndexIVFFlat.

Test Plan

• Verified fix with the reproduction snippet from Panorama merge_from corrupts merged vectors #5607 (all reconstructed vectors match ground truth and search distances are 0.0).
• Ran pytest tests/test_merge_index.py tests/test_flat_panorama.py (all passed).