Split scene_spec_generate_cmd under the complexity gate

[jmjava]

Summary

• Split scene_spec_generate_cmd into helpers so the command is lizard CCN 3 and radon A, within CCN ≤ 10 and radon ≤ C.
• Split the remaining src/docgen/cli.py commands that kept the file over the hotspot limit (image_generate_cmd, yaml_generate_cmd, benchmark, scene_compile, narration_generate, main, compose, _load_env). File max CCN is now 9. CLI behavior is unchanged.
• Benchmark stays green against src/docgen/benchmark_data/baseline.json. Complexity, NLOC, and mutation ceilings are unchanged.

Test plan

• pytest tests/ (903 passed; scratch-validate script passed on a Python that can import click)
• docgen benchmark meets baseline
• check-complexity.py passes
• check-hotspots.py passes (src/docgen/cli.py maxCCN 9, limit 10)
• CI lint, unit, benchmark, validate, mutation