#912 setup: show the GPU's PCIe link in step 1, warn on fewer lanes than the card has

Description

Step 1 (and `--check`) says the GPU's PCIe link, and warns when the card runs on fewer lanes than it has.

On an X99 board with an RTX 4060 Ti (`--check`, this branch):

```
=== Step 1: checking your PC ===
  [ok] GPU: NVIDIA GeForce RTX 4060 Ti, 16.0 GB VRAM, compute capability 8.9, driver 580.178.04
  [ok] RAM: 94 GB
  [ok] CPU: Intel(R) Xeon(R) CPU E5-2673 v3 @ 2.40GHz (AVX2)
  [ok] PCIe: 3.0 x8 (the card supports 4.0, the board 3.0) - up to ~8 GB/s to the GPU: long prompts are read slower than on a PCIe 4.0 x16 PC (their experts are copied over it); the engine measures the link at start for the output speed
```

and with a card on fewer lanes than it has:

```
  [!]  the GPU runs on 4 of its 16 PCIe lanes: is it in the right slot (the one wired x16), fully seated, and not sharing lanes with an M.2 drive? (the BIOS can say)
```

Why: the prompt path streams experts over the link, so its rate decides prompt speed (on this PC 714 tok/s for a
3,454-token prompt on Q2_0, against 2,650 at 32K on the RTX 5070), and a card in the wrong slot or sharing lanes is a
common and silent loss on older boards.

How: `nvidia-smi --query-gpu=pcie.link.gen.max,pcie.link.gen.gpumax,pcie.link.gen.hostmax,pcie.link.width.current,
pcie.link.width.max`. The current generation is not asked: an idle card drops to a lower one (this one reported gen 1
at idle). NVIDIA only (AMD gets no line); nothing changes when nvidia-smi does not answer.

Tests: `tools/test_setup_pcie.py` (4: parsing, an unparsable answer, a board-limited slow link, a fast link with no
extra text, a card on 4 of 16 lanes). All `tools/test_setup_*.py` pass.

🤖 Generated with [Claude Code](https://claude.com/claude-code)