feat: allow --shell on a per-command basis

[wyf027]

Summary

• Allow --shell/-S to be specified once for all commands or once per benchmark command (including reference), matching the pattern used by --prepare and --output.
• Each benchmark now uses its own executor and shell calibration when needed.

Fixes #850

Example

hyperfine \
  -n zsh -S zsh '[[ -n $ZSH_VERSION ]] && echo something in zsh' \
  -n bash -S bash '[[ -n $BASH_VERSION ]] && echo something in bash' \
  -n "no shell" -S none 'echo no shell'

Test plan

• cargo fmt
• cargo test (unit + integration)
• Added integration tests for per-command shell behavior and validation of shell count

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