fix: IVF GNO-IMI build fails recall threshold

[vsag-bot]

Summary

GNO-IMI now applies consistent cosine normalization during training, build/add routing, and search routing. A deterministic scale-invariance regression covers the failure mechanism.

Root Cause

GNO-IMI normalized cosine vectors during training and query routing but classified raw base vectors during build/add. This violated cosine scale invariance, assigned vectors to inconsistent buckets, and could lower recall below the functional-test threshold.

Changes

• Normalize cosine vectors before GNOIMIPartition build/add bucket classification.
• Add a fixed-seed unit test verifying identical routing for raw and normalized cosine vectors.

Validation

• pre-fix scale-invariance regression: passed - Reproduced different bucket assignments before applying the production fix.
• cmake --build build-ai-fix-local --target unittests --parallel 4: passed - Affected library and unit tests built successfully.
• unittests "[GNOIMIPartition]": passed - All 3 test cases and 9 assertions passed.
• unittests "GNO-IMI cosine build routing is scale invariant": passed - All 512 routing assertions passed.
• functests "(PR) IVF GNO-IMI Build": passed - All 38,620 assertions passed.
• clang-format-15 --dry-run --Werror; git diff --check: passed - No formatting or whitespace errors.
• clang-tidy-15: not_run - clang-tidy 15 is unavailable in this environment.

Residual Risks

• The full ASan suite and exact stochastic CI matrix were not run locally; the deterministic regression directly covers the normalization defect.
• The unrelated recoverable MemoryIO UBSan diagnostics reported in CI were not addressed.

Base branch: main

Fixes #2743

Prepared by vsag-bot after automatic effort ordering and AI repairability preflight.

[vsag-bot]

/label status/waiting-for-review
/waiting-on reviewer
/request-review @jiaweizone
/request-review @wxyucs
/request-review @inabao
/request-review @LHT129

[vsag-bot]

AI PR state: status/waiting-for-review
Current owner: @wxyucs
Reason: human approval is required
Head: 67bdb93

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