v0.35.1-rc0: create: support explicit model capabilities (#18708)

Summary

Add CAPABILITY declarations to Modelfiles and an additive capabilities field to create requests. Preserve declarations across GGUF and safetensors creation, inheritance, and Modelfile export. Require decision capability before scheduling System One requests instead of matching Qwen architecture/renderer metadata. Retain main's GGUF-only scoring restriction until the separate MLX runtime work lands. Extracted from the capability foundation in 36d46a0 on system_one_mlx; MLX scoring and manifest-list changes are intentionally separate.

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