feat: format empty partitions on system disk (v0.32.6)

Detect and offer to format empty (no filesystem) partitions on the system
disk. Adds IsSystemPartition() for granular per-partition safety checks
instead of blocking the entire system disk. Init wizard shows formatable
partitions with appropriate warnings. Add felhotest demo node to docs.

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

controller/internal/storage/safety_linux.go

@@ -5,6 +5,7 @@ package storage
 import (
 	"fmt"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"strings"
 	"syscall"
@@ -46,6 +47,68 @@ func IsSystemDisk(devicePath string) (bool, error) {
 	return rootDiskGroup == devDiskGroup, nil
 }
 
+// IsSystemPartition checks if a specific partition is a system partition
+// (/, /boot, /boot/efi, swap) or is currently mounted.
+// Unlike IsSystemDisk() which blocks the entire disk, this checks only the
+// individual partition — allowing non-system partitions on system disks to be formatted.
+func IsSystemPartition(partitionPath string) (bool, error) {
+	fstabPath := "/host-fstab"
+	if _, err := os.Stat(fstabPath); err != nil {
+		fstabPath = "/etc/fstab"
+	}
+
+	data, err := os.ReadFile(fstabPath)
+	if err != nil {
+		// If we can't read fstab, err on the side of caution
+		return true, fmt.Errorf("cannot read fstab: %w", err)
+	}
+
+	systemMounts := map[string]bool{"/": true, "/boot": true, "/boot/efi": true}
+
+	for _, line := range strings.Split(string(data), "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		fields := strings.Fields(line)
+		if len(fields) < 3 {
+			continue
+		}
+		source := fields[0]
+		mountPoint := fields[1]
+		fsType := fields[2]
+
+		if !systemMounts[mountPoint] && fsType != "swap" {
+			continue
+		}
+
+		var devPath string
+		if strings.HasPrefix(source, "UUID=") {
+			uuid := strings.TrimPrefix(source, "UUID=")
+			if out, err := exec.Command("blkid", "-U", uuid).Output(); err == nil {
+				devPath = strings.TrimSpace(string(out))
+			}
+		} else if strings.HasPrefix(source, "/dev/") {
+			devPath = source
+		}
+
+		if devPath == partitionPath {
+			return true, nil
+		}
+	}
+
+	// Also check if the partition is currently mounted
+	mounted, err := IsDeviceMounted(partitionPath)
+	if err != nil {
+		return false, err
+	}
+	if mounted {
+		return true, nil
+	}
+
+	return false, nil
+}
+
 // IsDeviceMounted checks if a device or any of its partitions is currently mounted.
 func IsDeviceMounted(devicePath string) (bool, error) {
 	data, err := os.ReadFile("/proc/mounts")