[MOD-14957] Normalize FLAT overwrites (1/3)

[dor-forer]

Describe the changes in the pull request

Single-value FLAT overwrites bypassed storage preprocessing. A cosine vector such as [2, 2, 2, 2] could replace an existing vector without being normalized. Run the replacement through preprocessForStorage, as insertion already does, while keeping the label's internal ID.

Stack 1/3, based on main. #1035 adds tiered SQ8 with an immediate backend; #1029 adds accumulation and deferred backend creation on top. Merge #1034, then #1035, then #1029.

Which issues this PR fixes

• An existing cosine-overwrite bug exposed while implementing MOD-14957, extracted from [MOD-14957] Add SQ8 accumulation and deferred backend creation (3/3) #1029.

Main objects this PR modified

• BruteForceIndex_Single::addVector and its cosine overwrite regression test.

Validation

Built and tested on SSH dorer-intel at c26aa251: all 138 test_bruteforce tests passed, including the new regression. No local build was run.

GitHub validation checked on 2026-09-07: the sanitizer check passed. The coverage job failed after its self-hosted runner lost contact with GitHub. The underlying cause is not yet diagnosed, and coverage validation remains incomplete.

Mark if applicable

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

---

Note

Low Risk
Small, localized fix to the overwrite branch in BruteForceIndex_Single::addVector; behavior aligns with existing insert preprocessing and is covered by a new regression test.

Overview
Single-label brute-force (FLAT) updates now run replacement vectors through preprocessForStorage before updateElement, matching the insert path in appendVector. Previously, re-adding under an existing label wrote raw input, so cosine indexes could store unnormalized values and return wrong distances.

A unit test asserts that overwriting a cosine label with [2,2,2,2] leaves one normalized vector in storage.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.42%. Comparing base (fd9fe8d) to head (22533ce).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1034      +/-   ##
==========================================
+ Coverage   97.40%   97.42%   +0.02%     
==========================================
  Files         141      141              
  Lines        8698     8699       +1     
==========================================
+ Hits         8472     8475       +3     
+ Misses        226      224       -2     

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