#455 setup: mark the Unsloth family NVIDIA only so far in the AMD menu, and point a mixed PC to its NVIDIA card (#429)

Description

Fixes #429

#429 was that on the HIP backend setup offered the Unsloth UD-Q4_K_XL and went on to the 111 GB download, although docs/UNSLOTH_Q4.md and the README say it needs an NVIDIA GPU. 3a73c40 already asks before the download now, so I dropped my hard stop and kept to that. Two small things are left:

- On the HIP backend the model menu marks the Unsloth family "[NVIDIA only so far]", next to the existing "[experimental]" mark, so it shows before you pick it.
- On a PC that also has an NVIDIA card, the stop before the download says to run setup without --backend hip to use the NVIDIA card, ahead of the 2-3-bit models and the --yes hint. On an AMD-only PC the message is unchanged.

NVIDIA output is unchanged.

Tests in tools/test_setup_unsloth.py were rewritten for this: the mixed PC hint with --backend hip (still no download), and the menu mark on an AMD-only PC with the backend auto-detected, with no mark on NVIDIA.

python -m unittest tools.test_setup_amd tools.test_setup_choices tools.test_setup_draft_vocab tools.test_setup_golden tools.test_setup_lowram tools.test_setup_pins tools.test_setup_prompts tools.test_setup_risk tools.test_setup_rope tools.test_setup_unsloth: 173 tests OK.