feat(bench): implement FRAMES dataset loader

[bug-ops]

Description

Implement the FRAMES (Google 2024) dataset loader and accuracy evaluator.

Part of epic #2827. See spec: .local/specs/zeph-bench/spec.md section 6.

Scope

• FramesLoader parsing FRAMES JSON schema into Vec<Scenario>
• Accuracy evaluator (exact match against gold answer)
• Download and cache to ~/.local/share/zeph/bench/frames/
• Unit tests with a synthetic fixture

Acceptance Criteria

• Loader parses official FRAMES schema
• Accuracy evaluator correct on fixture
• zeph bench download --dataset frames works