feat(bench): add zeph bench CLI subcommand (list, download, run, show)

[bug-ops]

Description

Add the bench top-level subcommand to the main zeph CLI using clap, gated on the bench feature.

Part of epic #2827. See spec: .local/specs/zeph-bench/spec.md section 9.

Scope

• zeph bench list — print table of supported datasets with cache status
• zeph bench download --dataset <name> — download and cache a dataset
• zeph bench run --dataset <name> --output <path> [--scenario <id>] [--provider <name>] [--baseline] [--resume] [--no-deterministic] — run a benchmark
• zeph bench show --results <path> — pretty-print a results.json file
• DatasetRegistry struct listing the 5 supported datasets with metadata
• Progress reporting to stderr (scenario N/M, current score)
• Exit code 1 on unknown dataset name or missing dataset cache

Acceptance Criteria

• zeph bench list prints all 5 dataset names
• zeph bench run --dataset unknown exits 1 with helpful message
• zeph bench run without --download when dataset not cached exits 1 with instruction
• CLI flags match spec section 9
• bench feature gates the subcommand at compile time