Expand BetaFilter for External SearchStrategy Implementations

[hailangx]

• [ y] Does this PR have a descriptive title that could go in our release notes?
• [n ] Does this PR add any new dependencies?
• [y ] Does this PR modify any existing APIs?
• [ y] Is the change to the API backwards compatible?
• [n ] Should this result in any changes to our documentation, either updating existing docs or adding new ones?

Reference Issues/PRs

What does this implement/fix? Briefly explain your changes.

The extra O generic is needed because the original 2-parameter form
SearchStrategy<Provider, T> defaults O to Provider::InternalId, which is u32. That means BetaFilter could only produce search results as u32 (internal IDs).

In diskann-garnet, the unfiltered path uses FullPrecision, which
implements SearchStrategy<Provider, T, O> for any O — so it can output
GarnetId directly into SearchResults. But BetaFilter was
locked to O = u32, forcing diskann-garnet to use the
FilteredSearchResults adapter to convert u32 → GarnetId after the
search.

By adding O to BetaFilter's impl:

impl<Provider, Strategy, T, I, O> SearchStrategy<Provider, T, O> for
BetaFilter<Strategy, I>
where
O: Send,
Strategy: SearchStrategy<Provider, T, O>,

BetaFilter becomes transparent to the output type — it delegates O to
whatever the inner strategy uses. If the inner strategy (FullPrecision)
can output GarnetId, then BetaFilter wrapping it can too. This
eliminates the need for FilteredSearchResults entirely.

The O: Send bound is required because SearchStrategy needs O: Send for
the search output buffer to work across async boundaries.

Any other comments?

Removed previous solution to reimport the beta's internal:
Why these types need to be public:

• BetaFilter's blanket impl (SearchStrategy<Provider, T>) hard-codes the output to the provider's internal ID
  (u32). Consumers with a different external ID type (like GarnetId) can't use BetaFilter directly with their output
  buffer — they're forced to write an adapter (FilteredSearchResults) that accepts u32 and manually converts.
• If BetaAccessor/BetaComputer/Unwrap are re-exported, consumers can write:
  impl<T: VectorRepr> SearchStrategy<GarnetProvider, [T], GarnetId>
  for BetaFilter<FullPrecision, u32>
• naming the associated types directly, and the conversion
  happens in the post-processor — no wrapper needed.

[Copilot]

Pull request overview

This PR expands the diskann-providers public API for search “layer” composition by re-exporting the internal BetaFilter helper types so external crates can implement SearchStrategy variants without building adapter wrappers.

Changes:

• Re-export BetaAccessor, BetaComputer, and Unwrap alongside BetaFilter from the layers module.

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[hildebrandmw]

Quick question: would it make sense to update BetaFilter to be friendlier to what you are trying to do? I also think there is potential with Naren's planned post-process changes that we may be able to shim a custom post-process layer to do the id translation.

[hailangx]

Quick question: would it make sense to update BetaFilter to be friendlier to what you are trying to do? I also think there is potential with Naren's planned post-process changes that we may be able to shim a custom post-process layer to do the id translation.
do you mean adding the 3-param impl so BetaFilter is transparent to the external ID type?

[hildebrandmw]

I guess my question is this: you are likely not going to be the only one who runs into this issue. If there is an obvious change to the current BetaFilter to enable this, then I'm all for that. Otherwise, we can make these public as a stopgap until the post-processing change.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.14%. Comparing base (6f8e8eb) to head (44fd210).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #793      +/-   ##
==========================================
- Coverage   89.16%   89.14%   -0.02%     
==========================================
  Files         431      431              
  Lines       78895    78895              
==========================================
- Hits        70343    70330      -13     
- Misses       8552     8565      +13     

Flag	Coverage Δ
miri	89.14% <ø> (-0.02%)	⬇️
unittests	89.14% <ø> (-0.03%)	⬇️

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Files with missing lines	Coverage Δ
...ders/src/model/graph/provider/layers/betafilter.rs	92.77% <ø> (ø)

... and 7 files with indirect coverage changes

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