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/verbose/verbose.go

@@ -0,0 +1,16 @@
+package verbose
+
+import (
+	"fmt"
+	"os"
+)
+
+// Enabled is set by the root command's --verbose flag.
+var Enabled bool
+
+// Printf prints to stderr if verbose mode is enabled.
+func Printf(format string, args ...any) {
+	if Enabled {
+		fmt.Fprintf(os.Stderr, "arca: "+format+"\n", args...)
+	}
+}