Support generic categories and map to MMLU-Pro

[tao12345666333]

Currently, only MMLU-Pro categories are supported for routing classification.

Proposal:

• Add support for generic, free-style categories in config.
• Provide a mapping layer that translates generic categories to the closest MMLU-Pro categories for classifier compatibility.
• Update docs and examples to show how to configure mappings.

Originally posted by @rootfs in #110 (comment)

[Xunzhuo]

/area bench
/priority P1
/milestone v0.1

[tao12345666333]

/assign