Remove try_into_vector_id conversion in prune.

[hildebrandmw]

Pruning did kind of a sneaky trick of reusing the output neighbor buffer as temporary scratch space for indices during prune. While this worked, it relied on usize variables being convertible to VectorId, which is kind of dubious because (1) we are moving away from VectorIds being convertible to primitive values and (2) it doesn't really provide a good guarantee on when the conversion will succeed or not.

This PR fixes that by using a dedicated scratch to store these indices.

While I originally introduced a third Vec in prune::{Scratch, Context}, I realized that all the auxiliary variables could be collapsed into a single State. Note that we get the extra 2-bytes for neighbor basically for free because even without it, padding would bring the size of State to 8 bytes.

Also, this fixes a potential bug where the max occlusion size could be set larger than u16::MAX and thus silently truncate via _ as u16 by enforcing a u16::MAX bound in diskann::graph::Config. This far exceeds any practical value anyone should be setting for this value so is unlikely to result in any breakage.

[Copilot]

Pull request overview

This PR refactors the pruning scratch-space layout to remove reliance on converting candidate indices into VectorId during pruning, replacing the separate occlude_factor / last_checked scratch vectors with a unified per-position State. It also tightens max_occlusion_size to a u16-bounded configuration to avoid silent truncation.

Changes:

• Replace prune scratch bookkeeping (occlude_factor, last_checked) with a single State vector and update pruning logic accordingly.
• Enforce max_occlusion_size <= u16::MAX at the configuration level and update tests/defaults.
• Adjust config conversion helpers (macro → traits/functions) and update experimental config accessors.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
diskann/src/graph/internal/prune.rs	Consolidates prune scratch state into a State struct and updates Scratch/Context accordingly.
diskann/src/graph/index.rs	Updates pruning implementation to use states and removes the try_into_vector_id conversion path.
diskann/src/graph/config/mod.rs	Changes max_occlusion_size storage to NonZeroU16, adds generic non-zero parsing helpers, and updates tests.
diskann/src/graph/config/experimental.rs	Updates to new ToNonZeroUsize helper usage.
diskann/src/graph/config/defaults.rs	Changes default MAX_OCCLUSION_SIZE type to NonZeroU16.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.84211% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.40%. Comparing base (6168ef0) to head (31fdca3).

Files with missing lines	Patch %	Lines
diskann/src/graph/index.rs	93.47%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1133   +/-   ##
=======================================
  Coverage   89.40%   89.40%           
=======================================
  Files         485      485           
  Lines       92079    92107   +28     
=======================================
+ Hits        82324    82351   +27     
- Misses       9755     9756    +1     

Flag	Coverage Δ
miri	89.40% <96.84%> (+<0.01%)	⬆️
unittests	89.06% <96.84%> (+<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/graph/config/experimental.rs	100.00% <100.00%> (ø)
diskann/src/graph/config/mod.rs	98.14% <100.00%> (-0.10%)	⬇️
diskann/src/graph/internal/prune.rs	62.50% <100.00%> (-1.79%)	⬇️
diskann/src/graph/index.rs	96.05% <93.47%> (-0.10%)	⬇️

... and 1 file with indirect coverage changes

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