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!