docs: update benchmark results

[loookashow]

Summary

Updates README and docs after the speaker post-processing changes:

• Refresh VoxConverse dev benchmark numbers: ~5.2% weighted DER, ~2.4% median DER
• Update speaker count summary: 117/216 exact, 175/216 within +/-1
• Document the current clustering pipeline: GMM BIC + silhouette refinement, spectral clustering, spherical reassignment, and timeline smoothing
• Add a limitation for speaker label switching / fragmentation on noisy real-world audio

Validation

• python -m ruff check .
• python -m ruff format --check .
• python -m mkdocs build --strict --site-dir /private/tmp/diarize-site

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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