Fix CSV export with --reference and parameter scans

[lawrence3699]

Fixes #852.

Before this change, CSV export took parameter column names from only the first benchmark result. When --reference ran before a parameter scan, that first result had no parameters, so the header stopped at the timing columns and the first parameterized row failed with:

CSV error: found record with 9 fields, but the previous record has 8 fields

This collects parameter names across all results and writes empty cells for rows that do not have a value, so reference rows can be exported alongside parameterized benchmarks.

Validation:

• cargo test export::csv::test_csv -- --nocapture
• cargo test export::csv::test_csv_with_reference_row_before_parameterized_rows -- --nocapture
• cargo fmt -- --check
• cargo run -- --runs 1 --reference 'sleep 1' --parameter-scan secs 2 3 --export-csv /tmp/hyperfine-852.csv 'sleep {secs}'

[Copilot]

Pull request overview

Fixes a CSV export edge case where --reference results (with no parameters) could appear before parameterized benchmark results, causing inconsistent CSV column counts and a serialization error.

Changes:

• Collect parameter names across all benchmark results to build a complete CSV header.
• Emit empty CSV cells for parameter columns that are not present on a given row (e.g., reference rows).
• Add a regression test covering the “reference row before parameterized rows” scenario.

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