fix(fit): prefer native MXFP4 quant path for gpt-oss-class MoE models

Context

PR #971 makes plan --quant Q4_K_M --profile ryzen-ai-max-plus-395 openai/gpt-oss-120b report ~50 tok/s (matching the #969 measured reference). Dynamic fit still picks Q8_0 on a 128 GB profile and estimates ~23.5 tok/s, because gpt-oss ships MXFP4-native weights and the Tier-1 scalable term prices them at quant_bpp("Q8_0").

Ask

• Teach quant selection / bpp that gpt_oss (and similar) prefer native MXFP4 / low-precision quants when available
• Surface a note on non-native quants so the estimate isn't silently optimistic/pessimistic
• Prefer scraper/runtime logic over hand-editing hf_models.json (per AGENTS.md)

Related: #969, #971