Fix IndexIVFFastScan reconstruct_from_offset method

[alisafaya]

Resolves issue #4089 - IndexIVFPQFastScan crashes with certain nlist values

The reconstruct_from_offset method in IndexIVFFastScan was incorrectly reconstructing vectors, causing crashes when the nlist parameter was not byte-aligned (e.g. 100 instead of 256).

The root cause was that the list_no (Voronoi cell number) was not being properly encoded into the code vector before passing it to the sa_decode function. This resulted in invalid list_no values being read in sa_decode, triggering the assertion failure 'list_no >= 0 && list_no < nlist' when nlist in some cases.

This PR fixes the issue with the following changes to reconstruct_from_offset:

1. Encode the list_no into the beginning of the code vector using the existing encode_listno method
2. Start the BitstringWriter after the coarse code portion of code (shifted by coarse_code_size() bytes)
3. Remove the residual centroid addition logic, as it is already handled in sa_decode

After these changes:

• Crashes no longer occur for any nlist value
• Reconstruction is now correct, matching the output of IndexIVFPQ

Fixes #4089

Please review and let me know if any changes are needed. Thanks!

[asadoughi]

Thanks for the contribution! Can you add unit tests, in Python should be sufficient, for the two cases you referenced?

• Crashes no longer occur for any nlist value
• Reconstruction is now correct, matching the output of IndexIVFPQ

[alisafaya]

I added unit tests, and fixed another bug in IndexIVFFastScan::reconstruct_orig_invlists().

The bug involves incorrect usage of invlists vs orig_invlists when getting the list size. The original code was using orig_invlists->list_size() which is incorrect since we want to read from the source invlists when getting codes and sizes. The fix changes it to use invlists->list_size() to be consistent with where we're reading the codes and IDs from.

[mdouze]

LGTM

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