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>

internal/unlock/unlock.go

@@ -8,6 +8,7 @@ import (
 
 	"git.wntrmute.dev/kyle/arca/internal/cryptsetup"
 	"git.wntrmute.dev/kyle/arca/internal/udisks"
+	"git.wntrmute.dev/kyle/arca/internal/verbose"
 )
 
 // Result holds the outcome of a successful unlock.
@@ -35,12 +36,16 @@ func New(client *udisks.Client, opts Options) *Unlocker {
 
 // Unlock tries each method in order and returns the result on first success.
 func (u *Unlocker) Unlock(dev *udisks.BlockDevice, methods []string) (*Result, error) {
+	verbose.Printf("unlock %s: methods %v", dev.DevicePath, methods)
 	var errs []error
 	for _, method := range methods {
+		verbose.Printf("trying method: %s", method)
 		res, err := u.tryMethod(dev, method)
 		if err == nil {
+			verbose.Printf("method %s succeeded", method)
 			return res, nil
 		}
+		verbose.Printf("method %s failed: %v", method, err)
 		errs = append(errs, fmt.Errorf("%s: %w", method, err))
 	}
 	return nil, fmt.Errorf("all unlock methods failed for %s:\n%w", dev.DevicePath, errors.Join(errs...))