Split CLI benchmark args

[yvette-carlisle]

Summary

• move ELF CLI benchmark clap argument models into a private child module
• keep crate-local crate::args::{BenchmarkArgs, BenchmarkCommand, BenchmarkReportArgs, BenchmarkRunArgs} imports working
• keep benchmark run/report task mapping behavior unchanged

Validation

• cargo make fmt-rust
• cargo make check-rust
• cargo make lint-vstyle
• cargo make lint-rust
• cargo make test-rust: 370 passed / 92 skipped
• cargo make check: 370 passed / 92 skipped
• cargo make test-rust-integration: 92 passed / 370 skipped
• cargo run -p elf -- benchmark --help
• cargo run -p elf -- benchmark run --dry-run --pretty
• cargo run -p elf -- benchmark report --dry-run --pretty
• cargo make check-trace-gate
• git diff --cached --check

Skeptic review found no blockers and recommended commit.