M7: add verbose mode for debugging

Add -v/--verbose persistent flag that prints debug info to stderr:
D-Bus connection status, token plugin directory discovery, unlock method
sequencing with per-method success/failure, and full cryptsetup command
lines including LD_LIBRARY_PATH.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cmd/root.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os"
 
+	"git.wntrmute.dev/kyle/arca/internal/verbose"
 	"github.com/spf13/cobra"
 )
 
@@ -12,6 +13,10 @@ var rootCmd = &cobra.Command{
 	Short: "Mount and unmount LUKS-encrypted volumes",
 }
 
+func init() {
+	rootCmd.PersistentFlags().BoolVarP(&verbose.Enabled, "verbose", "v", false, "print debug information to stderr")
+}
+
 func SetVersion(v string) {
 	rootCmd.Version = v
 }