fix: expose HYPERFINE_ITERATION to --prepare and --conclude (fixes #781)

[wyf027]

Summary

Fixes #781: $HYPERFINE_ITERATION is now set for --prepare and --conclude commands with the same iteration value as the corresponding benchmark/warmup run.

Implementation

Core change from #857 by @alexchenai (thank you!):

• #[derive(Clone, Copy)] on BenchmarkIteration
• Thread iteration through run_preparation_command / run_conclusion_command

This PR adds:

• Integration test hyperfine_iteration_env_in_prepare_and_conclude_commands
• CHANGELOG entry

Happy to close in favor of #857 if the author adds the test there — goal is to land a tested fix.

Test plan

• cargo test --locked (40 tests)
• Manual repro from ${HYPERFINE_ITERATION} is not visible in prepare or conclude commands #781:

  hyperfine --runs 2 --warmup 0 --show-output \
    --prepare 'echo prep-$HYPERFINE_ITERATION' \
    --conclude 'echo done-$HYPERFINE_ITERATION' \
    'echo run-$HYPERFINE_ITERATION'

[wyf027]

CI green (including Windows after gating the integration test to Unix). Ready for review — thanks!

[wyf027]

Friendly ping — @sharkdp noted on #781 this should be straightforward; CI is green on Unix/Windows. Based on #857 with an integration test. Thanks!

[wyf027]

Friendly ping — implements #781 (based on #857) with integration test; CI green on Unix and Windows. Thanks!

[wyf027]

Friendly ping — PR #874 still green on all platforms. Thanks for any review when you have time.

[wyf027]

Friendly ping — PR #874 (HYPERFINE_ITERATION in prepare/conclude) still CI-green. Thanks!

[wyf027]

@sharkdp Friendly ping on the hyperfine batch when you have time — all five PRs (#874–#878) are CI-green:

• fix: expose HYPERFINE_ITERATION to --prepare and --conclude (fixes #781) #874 — HYPERFINE_ITERATION in --prepare/--conclude (${HYPERFINE_ITERATION} is not visible in prepare or conclude commands #781)
• fix: use cargo_bin! macro in integration tests (fixes #844) #875 — cargo_bin! in tests (Tests rely on deprecated assert_cmd::cargo_bin which will break on future Cargo versions #844)
• fix: normalize forward slashes in paths for cmd.exe on Windows (fixes #818) #876 — Windows path slash normalization (Default execution on Windows fails when using forward slashes as directory separator #818)
• fix: include parameter columns in CSV when using --reference (fixes #852) #877 — CSV parameter columns with --reference (CSV error when using parameters with --reference #852)
• fix: clarify --reference with parameterized --prepare (fixes #847) #878 — --reference + parameterized prepare docs (Unclear how to use --reference with --prepare #847)

[wyf027]

Rebased onto latest master (merge commit 7079472); integration tests still pass locally.