feat: accept direct command arguments after separator

[Huzaifa-Asif]

Summary

• treat the executable and arguments after -- as one benchmark command
• preserve argument boundaries by passing the captured argv directly to std::process::Command
• keep the existing positional syntax unchanged for benchmarking multiple shell command strings
• document that the new form runs without an intermediate shell

Unlike the closed, unmerged #880, this does not join arguments into a command expression for execution. shell_words::join is used only to produce the human-readable benchmark name; the executed argv remains structured.

The branch now includes upstream master at e4a3213; the conflict resolution preserves the new metrics and default-shell CLI behavior while retaining direct-argument parameter detection. The regression test was adapted to the upstream removal of --time-unit without weakening its quoted-argument-boundary assertion.

Verification

• cargo fmt -- --check — passed on exact head 398f6a8
• cargo clippy --locked --all-targets -- -D warnings — passed with Rust 1.99.0
• cargo test --locked — 124 tests passed with Rust 1.99.0 (52 unit, 18 execution-order, 54 integration)
• focused regression verifies placeholders used in the executable and arguments are not reported as unused
• prior-head CI passed with the repository MSRV; current-head CI remains authoritative for platform and MSRV coverage
• git diff --check — passed

Fixes #802

AI assistance disclosure

OpenAI Codex assisted with command-path research, implementation, regression tests, conflict resolution, verification, and drafting this description.