Restore the ability to have Owned computers.

[Mark Hildebrand (hildebrandmw)]

Turns out #1247 was a little premature - we still have some code that is dependent on using Arc<FixedChunkPQTable>. Until a full replacement for PQ is in place, this is a stop-gap to enabled reference counted computers.

The main idea is to introduce

struct Shared<'a, T> {
    Arc(Arc<T>),
    Ref(&'a T),
}

as the container for the FixedChunkPQTable. Types that need to use Arc can use Shared<'static, T>. Existing code can continue to use Shared::Ref.

[Copilot]

🟡 Changes recommended

The new Shared unit test contains an invalid pointer comparison (shared.as_ptr() vs Arc::as_ptr()), which will not type-check and should be corrected before merging.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR reintroduces reference-counted ownership for PQ distance/query computers by adding a Shared<'a, T> container (borrowed &T or owned Arc<T>) and threading it through the PQ distance stack and relevant providers/benchmarks, restoring compatibility with code paths that still require Arc<FixedChunkPQTable>.

Changes:

• Add Shared<'a, T> (Arc-backed “owned” + borrowed ref) to pq::distance and update PQ distance/query computer APIs to accept it.
• Update PQ distance implementations (L2/IP/Cosine/Dynamic/Multi) and async providers to pass Shared instead of raw &FixedChunkPQTable.
• Adjust benchmark code to construct Shared when creating PQ query computers.

File summaries

File	Description
diskann-providers/src/model/pq/distance/multi.rs	Switch MultiTable to store Shared PQ tables and update tests/callers accordingly.
diskann-providers/src/model/pq/distance/mod.rs	Introduce the new Shared<'a, T> container and add unit tests.
diskann-providers/src/model/pq/distance/l2.rs	Update L2 PQ query computer to hold Shared parent table.
diskann-providers/src/model/pq/distance/innerproduct.rs	Update inner product PQ query computer to hold Shared parent table.
diskann-providers/src/model/pq/distance/dynamic.rs	Update dynamic PQ query/distance computer constructors and vtable calls to use Shared.
diskann-providers/src/model/pq/distance/cosine.rs	Update cosine PQ query computer to hold Shared parent table.
diskann-providers/src/model/graph/provider/async_/memory_quant_vector_provider.rs	Wrap PQ table with Shared::Ref when creating computers.
diskann-providers/src/model/graph/provider/async_/fast_memory_quant_vector_provider.rs	Wrap PQ table with Shared::Ref when creating computers.
diskann-providers/src/model/graph/provider/async_/experimental/multi_pq_async.rs	Update multi-PQ table construction to use Shared::Ref for schemas.
diskann-benchmark/src/exhaustive/product.rs	Construct Shared when creating PQ query computers in benchmarks.

Review details

• Files reviewed: 10/10 changed files
• Comments generated: 1
• Review effort level: Lite

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Codecov Report

❌ Patch coverage is 97.53086% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.43%. Comparing base (600c2b9) to head (6e59fa7).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
diskann-providers/src/model/pq/distance/mod.rs	92.85%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1382      +/-   ##
==========================================
- Coverage   91.55%   91.43%   -0.12%     
==========================================
  Files         521      522       +1     
  Lines      100302   100766     +464     
==========================================
+ Hits        91828    92132     +304     
- Misses       8474     8634     +160     

Flag	Coverage Δ
miri	91.43% <97.53%> (-0.12%)	⬇️
unittests	91.22% <97.53%> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
diskann-benchmark/src/exhaustive/product.rs	83.33% <ø> (ø)
...aph/provider/async_/experimental/multi_pq_async.rs	97.54% <100.00%> (+0.01%)	⬆️
...ovider/async_/fast_memory_quant_vector_provider.rs	97.84% <100.00%> (ø)
...ph/provider/async_/memory_quant_vector_provider.rs	97.43% <100.00%> (ø)
diskann-providers/src/model/pq/distance/cosine.rs	98.75% <100.00%> (ø)
diskann-providers/src/model/pq/distance/dynamic.rs	97.33% <100.00%> (+0.07%)	⬆️
...nn-providers/src/model/pq/distance/innerproduct.rs	100.00% <100.00%> (ø)
diskann-providers/src/model/pq/distance/l2.rs	100.00% <100.00%> (ø)
diskann-providers/src/model/pq/distance/multi.rs	98.03% <100.00%> (+0.02%)	⬆️
diskann-providers/src/model/pq/distance/mod.rs	92.85% <92.85%> (ø)

... and 43 files with indirect coverage changes

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