ax(logging): rename mu to mutex for predictable naming

AX Principle 1: abbreviated field names increase semantic ambiguity.
Logger.mu renamed to Logger.mutex — no comment required to understand it.

Co-Authored-By: Charon <charon@lethean.io>

pkg/logging/logger.go

@@ -42,7 +42,7 @@ func (l Level) String() string {
 
 // Logger provides structured logging with configurable output and level.
 type Logger struct {
-	mu        sync.Mutex
+	mutex     sync.Mutex
 	output    io.Writer
 	level     Level
 	component string
@@ -91,8 +91,8 @@ func (l *Logger) WithComponent(component string) *Logger {
 // SetLevel adjusts the minimum level for subsequent log calls.
 // logger.SetLevel(logging.LevelDebug)
 func (l *Logger) SetLevel(level Level) {
-	l.mu.Lock()
-	defer l.mu.Unlock()
+	l.mutex.Lock()
+	defer l.mutex.Unlock()
 	l.level = level
 }
 
@@ -100,8 +100,8 @@ func (l *Logger) SetLevel(level Level) {
 // current := logger.GetLevel()
 // if current == logging.LevelDebug { logger.SetLevel(logging.LevelInfo) }
 func (l *Logger) GetLevel() Level {
-	l.mu.Lock()
-	defer l.mu.Unlock()
+	l.mutex.Lock()
+	defer l.mutex.Unlock()
 	return l.level
 }
 
@@ -110,8 +110,8 @@ type Fields map[string]interface{}
 
 // log writes a log message at the specified level.
 func (l *Logger) log(level Level, msg string, fields Fields) {
-	l.mu.Lock()
-	defer l.mu.Unlock()
+	l.mutex.Lock()
+	defer l.mutex.Unlock()
 
 	if level < l.level {
 		return