[Sparse ANN] Bump neural-sparse-cpp submodule for DiskSeismic madvise fix

[zirui-song-18]

Bump neural-sparse-cpp for the DiskSeismic mmap madvise fix (MADV_RANDOM for per_block), fixing a large memory-constrained latency regression. Details can be seen in: opensearch-project/neural-sparse-cpp#55

This makes the madvise hint a function of the index's access pattern instead of a single global default:

• DiskSeismic (DiskSeismicIndex, DiskSeismicScalarQuantizedIndex) → MADV_RANDOM (point lookups; suppress readahead).
• shared (SeismicIndex, SeismicScalarQuantizedIndex) → MADV_HUGEPAGE (broad scans; fewer TLB entries), unchanged.

NSPARSE_MMAP_ADVISE still overrides the choice (an empty value is treated as unset, so it can't silently reinstate the old default).

Measured impact

From the launch benchmark (i4i.16xlarge, MS MARCO V1, 12 GB page-cache cap, THP=[madvise], top_n=5, k′=20, recall ≈ 0.92), same index and queries:

arm	advise	p50	p99
disk (per_block)	hugepage (old default)	6.529 ms	13.21 ms
disk (per_block)	random (new default)	0.157 ms	0.296 ms

• ~42× lower p50, ~45× lower p99. The old default was reading ~9,426 KiB per query for the ~53–1,055 KiB a query actually needs (~860× amplification over a 4 KiB page once RAID readahead compounds it). latency (0.157 ms) — the cap costs it nothing, using < 4.76 GB of page cache.
• Resident memory for the disk arm drops from > 30.9 GB to 10.94 GB (−65%); the previously reported "per_block needs ~3× more RAM than shared" was largely a hugepage artifact (real ratio ~1.6×).

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[github-actions]

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⛔ Hard block: Issues at High severity or above will block this PR from merging.

Path	Line	Severity	Description
jni/external/neural-sparse-cpp	1	high	Git submodule dependency updated to an unverified external commit (da0d43c -> 2da4627). This is a JNI native-code dependency that runs with full system privileges; the new commit hash must be verified against the upstream repository to confirm it matches the stated changelog purpose (mmap madvise fix) and contains no malicious native code.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.21%. Comparing base (7a24f0a) to head (49c4275).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #2005   +/-   ##
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  Coverage     84.20%   84.21%           
- Complexity     4239     4241    +2     
=========================================
  Files           318      318           
  Lines         14887    14887           
  Branches       2425     2425           
=========================================
+ Hits          12536    12537    +1     
  Misses         1485     1485           
+ Partials        866      865    -1     

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[github-actions]

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