fix(health): mark PCIe link downgrade as FAIL instead of WARN

The PCIe link health check was producing inconsistent verdicts: when the
negotiated link did not meet the GPU's expected Gen/Width (e.g. an H200
running at Gen4 instead of Gen5, or any GPU dropping below x16), the code
correctly flipped overall_pass to False — but recorded the per-GPU status
as "WARN" rather than "FAIL".

This mismatch broke the convention used by every other check in the
module (temperature, ECC, throttling), where FAIL is the only status
that drives overall_pass=False, and WARN is purely informational. As a
result the rendered Markdown / table output would show a yellow WARN
badge for the affected GPU while the overall Health Check verdict came
back red FAIL, leaving operators to wonder which signal to trust.

A PCIe link downgrade is not a soft warning — it halves H2D/D2H
bandwidth (Gen5 x16 ~64 GB/s -> Gen4 x16 ~32 GB/s), directly impacting
data loading, checkpoint I/O, and ZeRO/offload throughput. For an
acceptance-test tool this should be a hard failure, consistent with how
overall_pass already treats it.

Change: in modules/health_check.py, set status to "FAIL" (not "WARN")
when pcie_ok is False. This applies to both the known-GPU path
(Gen >= expected and Width >= 16) and the unknown-GPU fallback path
(Width >= 8). No behavioral change to overall_pass — only the per-GPU
status string is corrected so the table view, Markdown report, and the
overall verdict now agree.

modules/health_check.py

@@ -122,7 +122,7 @@ class HealthCheck:
                 pcie_ok = pw >= 8  # unknown GPU: just check width
             if not pcie_ok:
                 overall_pass = False
-            checks["pcie_link"] = {"gen": pg, "width": pw, "status": "PASS" if pcie_ok else "WARN"}
+            checks["pcie_link"] = {"gen": pg, "width": pw, "status": "PASS" if pcie_ok else "FAIL"}
 
             sm = self._safe_int(clock_sms[i] if i < len(clock_sms) else 0)
             mm = self._safe_int(clock_mems[i] if i < len(clock_mems) else 0)