Extend benchmark logging to include build and search compressor information

[tlwillke]

This extends the v1 experiments.csv schema with build_compressor and search_compressor columns so logged runs record the resolved compressor configurations used during index build and search. Values use the compressor’s printable toString() (e.g., ProductQuantization[...]) and are CSV-escaped to preserve commas. Search compressor logging is null-safe when compression is disabled (cv == null).

[github-actions]

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[ashkrisk]

LGTM. Eventually it would be nice to have compressor parameters separated by column, but we can look into it later.