[diskann-inmem] Add scalar quantization

[Mark Hildebrand (hildebrandmw)]

This adds support for scalar quantization (bits 1 through 8) to diskann-inmem, follow the pattern mostly set up in #1469. This does require the Quantizer trait to differentiate between distances for insert and distances for general queries due to the need to rescale queries for wiki-cohere embedding.

I've found several areas in diskann-quantization that need help:

1. Compression and distance methods should not panic on invalid vector lengths - they should return error types instead (that would remove some of the semi-redundant length checking in this PR).
2. diskann-quantization needs better support for asymmetrical compression. It's really a shame that we need to handle the somewhat sketch query rescaling for inner-products manually.

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

❌ Patch coverage is 87.80804% with 94 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.00%. Comparing base (1cac598) to head (e16d644).

Files with missing lines	Patch %	Lines
diskann-benchmark/src/index/inmem2.rs	70.65%	54 Missing ⚠️
diskann-inmem/integration/index/runner.rs	65.51%	30 Missing ⚠️
diskann-inmem/src/repr/quantized/scalar.rs	97.64%	9 Missing ⚠️
diskann-inmem/src/repr/quantized/mod.rs	98.95%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (87.80%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@                   Coverage Diff                   @@
##           mhildebr/inmem-pq-2    #1484      +/-   ##
=======================================================
- Coverage                92.03%   92.00%   -0.04%     
=======================================================
  Files                      590      591       +1     
  Lines                   119785   120486     +701     
=======================================================
+ Hits                    110243   110848     +605     
- Misses                    9542     9638      +96     

Flag	Coverage Δ
miri	92.00% <87.80%> (-0.04%)	⬇️
unittests	91.94% <87.80%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
diskann-inmem/integration/main.rs	82.71% <100.00%> (+0.66%)	⬆️
diskann-inmem/src/neighbors.rs	100.00% <100.00%> (ø)
diskann-inmem/src/repr/full.rs	97.07% <100.00%> (-0.01%)	⬇️
...ann-inmem/src/repr/internal/quantization/rerank.rs	98.01% <100.00%> (-0.03%)	⬇️
diskann-inmem/src/repr/quantized/product.rs	98.75% <100.00%> (-0.04%)	⬇️
diskann-inmem/src/repr/spherical.rs	97.43% <100.00%> (ø)
diskann-vector/src/distance/distance_provider.rs	98.60% <100.00%> (+0.01%)	⬆️
diskann-inmem/src/repr/quantized/mod.rs	98.52% <98.95%> (-0.03%)	⬇️
diskann-inmem/src/repr/quantized/scalar.rs	97.64% <97.64%> (ø)
diskann-inmem/integration/index/runner.rs	88.56% <65.51%> (-3.75%)	⬇️
... and 1 more

... and 4 files with indirect coverage changes

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