feat(health): flag NVMe PCIe links below capability (lane-sharing) — 0.38.0

check_pcie_links() warns when an NVMe drive negotiates fewer lanes than it
supports — almost always motherboard lane-sharing (a GPU/second card or another
M.2 stealing lanes), the case the user asked about — and reports speed-only
reductions as info (slower slot / idle ASPM). GPU is excluded: NVIDIA drops its
PCIe gen+width at idle, so a snapshot would false-alarm. Reuses inventory
read_link/nvme_controllers (refactored to public). Wired into run_health_checks;
+tests. Folded into the 0.38.0 PCIe work.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -12,6 +12,11 @@ release tag (so the auto-updater, D18, can compare versions).
   (`current/max_link_speed` + width). If a drive negotiates below its capability (a slower M.2
   slot, lane-sharing, or a downtrain) it's flagged: `PCIe Gen3 x4 (capable of Gen4 x4)`. So you
   can confirm a Gen4 SSD is actually in a Gen4 slot. (SATA disks show no PCIe link.)
+- **System Health flags downtrained NVMe links.** A new check warns when an NVMe drive negotiates
+  fewer PCIe lanes than it supports (almost always motherboard **lane-sharing** — a GPU/second
+  card or another M.2 stealing lanes) and notes speed-only reductions as info (a slower slot or
+  idle ASPM). The GPU is deliberately excluded — NVIDIA drops its PCIe gen/width at idle, so a
+  snapshot would false-alarm.
 
 ## [0.37.1] - 2026-05-22
 ### Fixed