Deduplicate virtual start point edges during disk serialization

[juchen-ms (partychen)]

• Does this PR have a descriptive title that could go in our release notes?
• Does this PR add any new dependencies? No.
• Does this PR modify any existing APIs? No.
• Is the change to the API backwards compatible? Yes.
• Should this result in any documentation changes? No.

Reference Issues/PRs

N/A

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

The in-memory graph uses a virtual start point ID, while the serialized disk graph must replace it with the actual medoid ID.

Previously, this replacement happened after adjacency-list uniqueness had already been established. If a list contained both the virtual start point and the actual medoid, the remapping produced a duplicate medoid edge. On the medoid's own adjacency list, it could also produce a self-loop.

This PR updates DiskAdaptor to:

• remap the virtual start point to the actual medoid;
• preserve the original neighbor order;
• retain only the first resulting medoid edge;
• remove self-loops after remapping.

It does not backfill removed entries with arbitrary neighbors, so affected adjacency lists may contain one fewer unique edge.

The regression test covers both a remapping collision and the medoid self-loop case. The expected disk-index fixture is updated for the corrected serialized graph.

Any other comments?

The in-memory adjacency list is already unique before serialization, so only the alias introduced by virtual-start-point remapping needs post-remap deduplication. This keeps the change limited to disk serialization and does not alter graph construction or pruning behavior.

[Copilot]

Pull request overview

This PR fixes a disk-serialization edge case in diskann-providers where remapping the in-memory “virtual start point” ID to the actual medoid ID could introduce duplicate medoid edges and (on the medoid’s own adjacency list) create a self-loop in the serialized disk graph. The change is scoped to the disk-graph adaptor used during serialization and is validated by a new regression test plus an updated disk-index fixture.

Changes:

• Update DiskAdaptor::get_adjacency_list to remap the virtual start point to the actual start point while preserving neighbor order, deduplicating the resulting medoid edge (keep first), and removing self-loops after remapping.
• Add a unit test covering both the remapping collision (duplicate medoid) and medoid self-loop scenarios.
• Update the expected disk-index fixture (Git LFS pointer) to match the corrected serialized graph output.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
diskann-providers/src/model/graph/provider/async_/simple_neighbor_provider.rs	Adjusts disk-serialization adjacency list remapping to deduplicate the remapped medoid edge and remove self-loops; adds a regression test for both failure modes.
test_data/disk_index_build/truth_sift_learn_R4_L50_disk.index	Updates the expected on-disk index fixture pointer to reflect the corrected serialization output.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.53%. Comparing base (860cf47) to head (dc9b2b7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1350      +/-   ##
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+ Coverage   91.55%   92.53%   +0.97%     
==========================================
  Files         521      521              
  Lines      100347   100371      +24     
==========================================
+ Hits        91877    92876     +999     
+ Misses       8470     7495     -975     

Flag	Coverage Δ
miri	92.53% <100.00%> (+0.97%)	⬆️
unittests	92.49% <100.00%> (+1.25%)	⬆️

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

Files with missing lines	Coverage Δ
.../graph/provider/async_/simple_neighbor_provider.rs	94.09% <100.00%> (-2.63%)	⬇️

... and 43 files with indirect coverage changes

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