[flat] Post-processing support and use DataProvider::InternalId

[arkrishn94]

This PR adds support to do post-processing in the FlatIndex::knn_search via the diskann::glue::SearchPostProcess trait; which is the same interface used by the graph search API.

In addition, I had left a hook in the flat search SearchStrategy and DistancesUnordered to support ID types that were not convertible to and from scalar-like values. With #1145, that constraint is now gone in VectorId and we are free to correctly tie the ID type in the DistancesUnordered implementation used by the flat index to the InternalId of the underlying DataProvider. This means:

• DistancesUnordered now has HasId as a super-trait.
• SearchStrategy no longer needs Id as an associated type.

Testing

Added testing in the test module to make sure post-processing works as it should via a custom post-processor that only keeps even ids. Added some extra code in the test::harness module just to make the dispatch to post-processing generic enough so I can enumerate easily over cases.

For reviewers: Review the flat\strategy.rs changes first, then flat\index.rs. Optionally, then, review the testing code.

[Copilot]

Pull request overview

This PR extends FlatIndex::knn_search to support post-processing via the shared diskann::graph::glue::SearchPostProcess interface, aligning flat search with the graph search pipeline. It also simplifies flat search ID plumbing by tying DistancesUnordered/SearchStrategy directly to the provider’s DataProvider::InternalId (and making DistancesUnordered require HasId).

Changes:

• Add post-processing support to FlatIndex::knn_search via SearchPostProcess.
• Simplify flat-search ID typing by removing SearchStrategy::Id and making DistancesUnordered: HasId, using P::InternalId end-to-end.
• Extend flat test harness/tests to validate post-processing behavior (including an “even IDs only” post-processor).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
diskann/src/flat/test/provider.rs	Updates test visitor/strategy implementations for the new HasId-based ID wiring.
diskann/src/flat/test/harness.rs	Adds an oracle/processor abstraction and post-processing test utilities.
diskann/src/flat/test/cases/flat_knn_search.rs	Updates baseline flat knn test to use the new harness API (processor-aware).
diskann/src/flat/strategy.rs	Makes DistancesUnordered require HasId and ties SearchStrategy visitor IDs to P::InternalId.
diskann/src/flat/mod.rs	Updates module docs to reflect the shared post-processing interface.
diskann/src/flat/index.rs	Adds the SearchPostProcess hook to FlatIndex::knn_search and updates tests accordingly.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.47%. Comparing base (de5ac3c) to head (512427e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1160      +/-   ##
==========================================
+ Coverage   89.46%   89.47%   +0.01%     
==========================================
  Files         487      487              
  Lines       92106    92155      +49     
==========================================
+ Hits        82401    82455      +54     
+ Misses       9705     9700       -5     

Flag	Coverage Δ
miri	89.47% <100.00%> (+0.01%)	⬆️
unittests	89.12% <100.00%> (+0.01%)	⬆️

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

Files with missing lines	Coverage Δ
diskann/src/flat/index.rs	97.77% <100.00%> (+0.29%)	⬆️
diskann/src/flat/strategy.rs	96.89% <ø> (ø)
diskann/src/flat/test/cases/flat_knn_search.rs	94.38% <100.00%> (+0.26%)	⬆️
diskann/src/flat/test/harness.rs	100.00% <100.00%> (ø)
diskann/src/flat/test/provider.rs	73.14% <ø> (ø)

... and 2 files with indirect coverage changes

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