mikemccand opened a new pull request, #13874:
URL: https://github.com/apache/lucene/pull/13874

   This PR:
   
     1. Fixes a pre-existing BWC testing bug where our `int8_hnsw*.zip` bwc 
test files failed to actually use the scalar quantization codec ... they were 
just ordinary `float32` HNSW since they were using Lucene's default codec to 
write the bwc indices.  This is why no bwc tests failed with my original PR 
(#13767), sigh.  I also renamed these files and the test class itself to 
`int7*` since they are actually all 7 bit not 8 bit quantization.
     1. Regenerates the bwc indices for 9.10.0, 9.11.0, and 9.11.1 (the binary 
zip files).  This entailed checking out each release tag for these releases, 
backporting the fix above, regenerating the zip files and copying them into 
this PR.  It was a bit tricky for 9.10.0 ([see details 
here](https://github.com/apache/lucene/issues/13867#issuecomment-2400927997)).
     1. Fixes the actual bug from #13767 using the suggested fix from @parnmatt 
(thank you!).  The bug happened because scalar quantized HNSW indices written 
with 9.11.0 or 9.11.1 claimed `compress=true` (written into the index) yet 
vector compression is silently `false` unless `bits` is 7 or 8.  This trap is 
fixed for newly created indices in 9.12.0, but 9.11.x indices bake in the trap.
   
   I confirmed that after the 2nd step above, the bwc test was indeed failing 
properly (because of the #13867 bug), and then upon applying the fix in the 
third step above, bwc tests (and all tests) now pass.
   
   Thank you @neio-ionut for discovering and opening this issue and @parnmatt 
and @benwtrent for digging and proposing the root cause fix.
   
   Closes #13867 


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