(new): add log level helpers and HTTP method levels

- move LogLevel and predefined levels into levels.go
- add level-specific constructors for info/warn/error/fatal/debug
- add predefined HTTP method log levels and LogLevelForMethod
- unify Logger Print/Println internals and add Printf
- improve GoDoc coverage for exported APIs

io.go

@@ -83,9 +83,7 @@ func (l *Logger) Debugln(m ...any) {
 	l.Println(DEBUG, m...)
 }
 
-// Print write message without trailing "\n"
-// Good for database
-func (l *Logger) Print(level LogLevel, m ...any) {
+func (l *Logger) print(level LogLevel, newline bool, m ...any) {
 	if l.level.n < level.n {
 		return
 	}
@@ -94,30 +92,10 @@ func (l *Logger) Print(level LogLevel, m ...any) {
 	}
 
 	tb := getFullTraceback(1)
-	for _, writer := range l.writers {
-		if writer == nil {
-			continue
-		}
-		err := writer.Print(level, l.prefix, tb, l.replaceAll(m...)...)
-		if err != nil {
-			l.reportWriterError(err)
-		}
+	messages := l.replaceAll(m...)
+	if newline {
+		messages = append(messages, any("\n"))
 	}
-}
-
-// Println
-// Docker requires "\n" at end to write to log.
-// print not work for docker, otherwise it will work and write into stdout
-func (l *Logger) Println(level LogLevel, m ...any) {
-	if l.level.n < level.n {
-		return
-	}
-	if len(l.writers) == 0 {
-		return
-	}
-
-	tb := getFullTraceback(1)
-	messages := append(append(make([]any, 0, len(m)+1), l.replaceAll(m...)...), "\n")
 	for _, writer := range l.writers {
 		if writer == nil {
 			continue
@@ -129,6 +107,17 @@ func (l *Logger) Println(level LogLevel, m ...any) {
 	}
 }
 
+// Print logs a message without appending a trailing newline semantic.
+func (l *Logger) Print(level LogLevel, m ...any) { l.print(level, false, m...) }
+
+// Println logs a message and appends a trailing newline semantic for writers that need it.
+func (l *Logger) Println(level LogLevel, m ...any) { l.print(level, true, m...) }
+
+// Printf formats according to a format specifier and logs the resulting message.
+func (l *Logger) Printf(level LogLevel, format string, args ...any) {
+	l.print(level, false, fmt.Sprintf(format, args...))
+}
+
 // reportWriterError writes internal writer failures directly to stderr to avoid
 // re-entering the same logger path that just failed.
 func (l *Logger) reportWriterError(err error) {