Add USB export drive and LiveCD-in-RAM checks to Runtime Health

- schema: add ToRAMStatus and USBExportPath fields to RuntimeHealth
- platform/runtime.go: collectToRAMHealth (ok/warning/failed based on
  IsLiveMediaInRAM + toramActive) and collectUSBExportHealth (scans
  /proc/mounts + lsblk for writable USB-backed filesystems)
- pages.go: add USB Export Drive and LiveCD in RAM rows to the health table

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

audit/internal/schema/hardware.go

@@ -22,6 +22,10 @@ type RuntimeHealth struct {
 	CUDAReady     bool                   `json:"cuda_ready,omitempty"`
 	NvidiaGSPMode string                 `json:"nvidia_gsp_mode,omitempty"` // "gsp-on", "gsp-off", "gsp-stuck"
 	NetworkStatus string                 `json:"network_status,omitempty"`
+	// ToRAMStatus: "ok" (ISO in RAM), "warning" (toram not active), "failed" (toram active but copy failed)
+	ToRAMStatus   string `json:"toram_status,omitempty"`
+	// USBExportPath: mount point of the first writable USB drive found, empty if none.
+	USBExportPath string `json:"usb_export_path,omitempty"`
 	Issues        []RuntimeIssue         `json:"issues,omitempty"`
 	Tools         []RuntimeToolStatus    `json:"tools,omitempty"`
 	Services      []RuntimeServiceStatus `json:"services,omitempty"`