feat: serial:/size: external matching, exclude Hyprland FALLBACK output

- external list now accepts serial: and size: prefixes; desc: matches
  serial too. Fixes docked-mode deadlock where Hyprland's synthetic
  FALLBACK output (created when the built-in is disabled by a stale
  docked config) was treated as a real external monitor, leaving the
  daemon stuck in docked after undocking.
- modes name resolution gains serial: prefix (resolveSerial).
- add -log-level flag for observability.
- add unit tests for external matching and name resolution.
- update README and example config.

internal/backend/interface.go

@@ -117,19 +117,20 @@ type Backend interface {
 	Close() error
 }
 
-// resolveMonitorNames converts desc: or size: prefixed monitor config names
-// to the actual connector names by matching against the current set of
-// connected monitors.
+// resolveMonitorNames converts desc:, serial:, or size: prefixed monitor
+// config names to the actual connector names by matching against the current
+// set of connected monitors.
 //
 // Matching logic:
 //   - "desc:text" — substring match against description or serial
+//   - "serial:text" — substring match against serial
 //   - "size:WxH" — match by exact pixel dimensions (e.g. "size:2560x1440")
 //   - "size:WxH@R" — match by dimensions + refresh rate (e.g. "size:2560x1440@165")
 //   - plain names are returned as-is
 func resolveMonitorNames(ctx context.Context, monitors []MonitorConfig, getMonitors func(context.Context) ([]MonitorInfo, error), logger *slog.Logger) ([]MonitorConfig, error) {
 	var needsResolution bool
 	for _, m := range monitors {
-		if strings.HasPrefix(m.Name, "desc:") || strings.HasPrefix(m.Name, "size:") {
+		if strings.HasPrefix(m.Name, "desc:") || strings.HasPrefix(m.Name, "serial:") || strings.HasPrefix(m.Name, "size:") {
 			needsResolution = true
 			break
 		}
@@ -148,6 +149,8 @@ func resolveMonitorNames(ctx context.Context, monitors []MonitorConfig, getMonit
 		switch {
 		case strings.HasPrefix(m.Name, "desc:"):
 			resolved[i] = resolveDesc(m, current, logger)
+		case strings.HasPrefix(m.Name, "serial:"):
+			resolved[i] = resolveSerial(m, current, logger)
 		case strings.HasPrefix(m.Name, "size:"):
 			resolved[i] = resolveSize(m, current, logger)
 		default:
@@ -161,6 +164,38 @@ func resolveMonitorNames(ctx context.Context, monitors []MonitorConfig, getMonit
 	return resolved, nil
 }
 
+// resolveSerial resolves a serial: prefixed name to a connector name.
+func resolveSerial(m MonitorConfig, current []MonitorInfo, logger *slog.Logger) MonitorConfig {
+	needle := strings.TrimPrefix(m.Name, "serial:")
+	var matches []MonitorInfo
+	for _, mi := range current {
+		if strings.Contains(mi.Serial, needle) {
+			matches = append(matches, mi)
+		}
+	}
+
+	switch len(matches) {
+	case 0:
+		// Return as-is so the caller sees the empty name.
+		m.Name = ""
+		return m
+	case 1:
+		logger.Debug("resolved serial to connector",
+			"serial", needle, "connector", matches[0].Name)
+		m.Name = matches[0].Name
+		return m
+	default:
+		var names []string
+		for _, mat := range matches {
+			names = append(names, mat.Name)
+		}
+		logger.Warn("serial:%q matches multiple monitors: %s — use a more specific identifier",
+			needle, strings.Join(names, ", "))
+		m.Name = ""
+		return m
+	}
+}
+
 // resolveDesc resolves a desc: prefixed name to a connector name.
 func resolveDesc(m MonitorConfig, current []MonitorInfo, logger *slog.Logger) MonitorConfig {
 	needle := strings.TrimPrefix(m.Name, "desc:")