fix(pcie): stop misreporting empty/vanished slots as speed degradation

pcie-link's retrain check compared post-retrain speed against a port's
max capability without checking whether anything was actually seated
downstream. A switch downstream port with no card plugged in retrains
to zero lanes but still reports a reset-state speed, so it was
misreported as "retrained to Gen1, capable of Gen5" — indistinguishable
from a real degraded link.

Now a port that was already empty before the retrain is skipped (same
treatment as a disabled device: legitimately unpopulated slots are
common and shouldn't fail the check), while a device that answered
before the retrain and disappeared right after it still fails, with an
honest "no device detected downstream" message instead of a fabricated
speed reading.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

audit/internal/platform/pcie_link_check.go

@@ -35,6 +35,7 @@ type pcieLinkFinding struct {
 	Width       int
 	MaxWidth    int
 	Degraded    bool
+	NotPresent  bool // true when the slot trained to zero lanes: nothing is plugged in (or it fell off the bus), not a speed regression
 }
 
 // RunPCIeLinkCheckPack forces every enabled PCIe device to retrain its link
@@ -139,6 +140,20 @@ func retrainAndSamplePCIeDevice(ctx context.Context, verboseLog, bdf string, log
 		return f
 	}
 
+	if width == 0 {
+		// A downstream switch/root port with nothing seated reads zero
+		// trained lanes even before we touch it. Plenty of legitimate
+		// configs leave slots like this unpopulated (not every server ships
+		// every NIC/riser slot filled), so this is not by itself evidence
+		// of anything wrong — retraining an empty slot can't produce a
+		// meaningful speed reading, and there's no baseline here to say
+		// "this used to have a card." Skip it exactly like a disabled
+		// device: nothing to verify, no reason to fail the run over it.
+		f.NotPresent = true
+		f.Skipped = "no device present downstream (empty slot/riser — nothing to retrain)"
+		return f
+	}
+
 	if err := retrainPCIeLink(ctx, verboseLog, bdf, logFunc); err != nil {
 		f.Skipped = "retrain failed: " + err.Error()
 		f.AfterSpeed = before
@@ -151,6 +166,17 @@ func retrainAndSamplePCIeDevice(ctx context.Context, verboseLog, bdf string, log
 
 	after, _ := readPCIeSysfsString(bdf, "current_link_speed")
 	widthAfter, _ := readPCIeSysfsInt(bdf, "current_link_width")
+	if widthAfter == 0 {
+		// The device answered before the retrain but is gone immediately
+		// after it (fell off the bus mid-check) — unlike the pre-retrain
+		// case above, this had a live link a moment ago, so it's worth
+		// surfacing rather than silently skipping.
+		f.AfterSpeed = after
+		f.Width = widthAfter
+		f.NotPresent = true
+		f.Degraded = true
+		return f
+	}
 	f.AfterSpeed = after
 	f.Width = widthAfter
 	f.Degraded = after != maxSpeed
@@ -315,10 +341,16 @@ func renderPCIeLinkCheckSummary(findings []pcieLinkFinding) string {
 		fmt.Fprintln(&b, "overall_status=FAILED")
 		var reasons []string
 		for _, f := range findings {
-			if f.Degraded {
-				reasons = append(reasons, fmt.Sprintf("%s (%s): retrained to %s, capable of %s",
-					f.BDF, nonEmptyOr(f.Description, "unknown device"), f.AfterSpeed, f.MaxSpeed))
+			if !f.Degraded {
+				continue
 			}
+			if f.NotPresent {
+				reasons = append(reasons, fmt.Sprintf("%s (%s): no device detected downstream (link down / empty slot or riser, capable of %s)",
+					f.BDF, nonEmptyOr(f.Description, "unknown device"), f.MaxSpeed))
+				continue
+			}
+			reasons = append(reasons, fmt.Sprintf("%s (%s): retrained to %s, capable of %s",
+				f.BDF, nonEmptyOr(f.Description, "unknown device"), f.AfterSpeed, f.MaxSpeed))
 		}
 		fmt.Fprintf(&b, "warnings=%s\n", strings.Join(reasons, "; "))
 	} else {
@@ -340,7 +372,10 @@ func renderPCIeLinkCheckReport(findings []pcieLinkFinding) string {
 			continue
 		}
 		verdict := "OK"
-		if f.Degraded {
+		switch {
+		case f.NotPresent:
+			verdict = "FELL OFF BUS"
+		case f.Degraded:
 			verdict = "DEGRADED"
 		}
 		fmt.Fprintf(&b, "    %s: before=%s after=%s max=%s width=%d/%d\n",