Harden MLX multimodal safety defaults

[brianmeyer]

Summary

• serialize heavy MLX multimodal work and keep native MLX video off the default path
• add safer frame/pixel budgets, crash-safe smoke controls, and RSS telemetry
• degrade video query/reranker flows to caption or frame-based fallbacks instead of risky native video preprocessing

Validation

• python3 -m pytest -q tests/test_mlx_reranker_prompt.py tests/test_search_pipeline.py tests/test_cross_modal_benchmark_defs.py tests/test_query_expansion.py tests/test_config_tools.py
• python3 benchmarks/cross_modal_ablation.py --dry-run --backend mlx --smoke-profile safe --expansion-profile caption_only --rss-limit-mb 6144

Linear

• REC-202
• REC-203
• REC-204
• REC-205
• REC-206
• REC-212