Collect IOMMU group per PCIe device from sysfs

Reads the iommu_group symlink for each BDF and exposes the group number
as iommu_group in the hardware snapshot JSON.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

audit/internal/schema/hardware.go

@@ -211,6 +211,7 @@ type HardwarePCIeDevice struct {
 	Firmware               *string        `json:"firmware,omitempty"`
 	MacAddresses           []string       `json:"mac_addresses,omitempty"`
 	Present                *bool          `json:"present,omitempty"`
+	IOMMUGroup             *int           `json:"iommu_group,omitempty"`
 	Telemetry              map[string]any `json:"-"`
 }