[SVS][PoC] 2-steps vectors update in SVSTiered

[rfsaliev]

Proof-of-concept for 2-steps SVS index update

This PR is the PoC to demonstrate potential Tiered SVS index update performance improvements by using 2-steps index update capability implemented in intel/ScalableVectorSearch#348.

Which issues this PR fixes

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2. MOD...

Main objects this PR modified

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Mark if applicable

• This PR introduces API changes
• This PR introduces serialization changes

---

Note

Medium Risk
Tiered update locking and new SVS nightly binaries affect index correctness and build reproducibility; PoC status means the two-phase API may still evolve.

Overview
PoC for a two-phase SVS backend update so tiered flat→backend moves can do heavy work under a shared lock and only commit under an exclusive lock, matching upstream add_points_compute_changes / add_points_commit.

SVSIndexBase gains makeAddVectorsChanges and applyAddVectorsChanges (with SVSChangesHandler holding preprocessed vectors and computed changes). TieredSVSIndex::updateSVSIndex uses that path when the backend already exists instead of calling addVectors in one critical section.

CMake switches prebuilt SVS URLs from v0.3.1 to nightly *-pr330.tar.gz artifacts and drops the glibc 2.26 GCC binary path; the minimum glibc message for GCC is now 2.28.

For non-multi indices, topKQuery uses the single-query MutableVamanaIndex scratch search path instead of batch QueryResult search to cut overhead on one query.

Benchmarks add TopKSearchDuringUpdate to stress parallel TopK while a background tiered update runs.

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[codecov]

Codecov Report

❌ Patch coverage is 91.80328% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.08%. Comparing base (dd6879e) to head (97173eb).

Files with missing lines	Patch %	Lines
src/VecSim/algorithms/svs/svs.h	91.22%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #988      +/-   ##
==========================================
- Coverage   97.12%   97.08%   -0.05%     
==========================================
  Files         141      141              
  Lines        8245     8291      +46     
==========================================
+ Hits         8008     8049      +41     
- Misses        237      242       +5     

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[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 2 potential issues.

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[cursor]

TopK ignores requested k

High Severity

For non-multi SVS indices, topKQuery now uses the scratch single-query search path with VamanaSearchParameters from joinSearchParams, but only grows the scratch buffer to k. It does not align the search window with k. With the default search window of 10, a request for more than 10 neighbors can return far fewer results than before the batch QueryResult path.

 

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