fix: release SuperKMeans sample buffer after rotation

[marcelo-cjl]

Summary

issue: #1833

• release the temporary SuperKMeans subsample buffer immediately after rotation populates X_tilde
• avoid retaining the buffer during centroid initialization and iterative clustering
• keep the vendored FAISS implementation aligned with upstream PR fix: release SuperKMeans sample buffer after rotation facebookresearch/faiss#5648

Memory result

One build run on Cohere 1M x 768 FP32 with SCANN (nlist=1024, sub_dim=2, with_raw_data=false, 16 threads):

• SuperKMeans disabled: 6369.7 MiB average RSS
• SuperKMeans before this change: 6749.1 MiB average RSS
• SuperKMeans with this change: 6425.4 MiB average RSS
• optimized vs. unoptimized: -323.7 MiB average RSS (-4.8% overall; -8.5% of build RSS above baseline)

The sampled buffer is 768 MiB. The full-build average reduction is smaller because that buffer remains necessary during sampling and rotation.

Test plan

• make WITH_UT=True BUILD_DIR=<workspace>/build/main/ut
• knowhere_tests '[superkmeans],Test SuperKMeans*' (3 test cases, 44 assertions passed)
• git diff --check

Repository-wide pre-commit was also run. Its basic checks passed; clang-format reported existing formatting differences in unrelated files. This vendored path is excluded by that hook.

Closes #1833

[mergify]

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[foxspy]

/lgtm

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