fix: include parameter columns in CSV header when using --reference

[YoshKoz]

Fixes #852

Problem

When --reference and --parameter-scan are combined, the CSV export fails:

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

The root cause: the CSV header was built from results.first(), which is the reference command. The reference has an empty parameters map, so no parameter_* columns were written to the header. The subsequent parameterised rows have one extra field per parameter, causing a field-count mismatch.

Fix

Two changes to src/export/csv.rs:

1. Header: find the first result that actually has parameters (results.iter().find(|r| !r.parameters.is_empty())) instead of blindly using .first().

2. Data rows: iterate over the collected parameter names in order and look up each value with a fallback to empty string, so the reference row emits empty strings for parameter columns rather than zero fields.

Expected CSV output with --reference "sleep 1" --parameter-scan secs 2 3:

command,mean,stddev,median,user,system,min,max,parameter_secs
sleep 1,1.003,...,         ← reference: empty parameter column
sleep 2,2.001,...,2
sleep 3,3.001,...,3

A regression test (test_csv_with_reference) is included.