(new): add threshold-based log level filtering and docs

- add threshold-aware LogLevel constructors
- add Logger.SetThresholdMode and SameThreshold
- preserve legacy SameLevel behavior for compatibility
- extend tests for threshold mode and compatibility
- update README and release notes for v2.2.0

logger.go

@@ -18,12 +18,14 @@ type MethodTraceback struct {
 
 // Logger routes log records to one or more configured writers.
 type Logger struct {
-	prefix    string
-	level     LogLevel
+	prefix string
+	level  LogLevel
+
 	writers   []LoggerWriter
 	replacers []replacer
 
 	jsonPretty bool
+	threshold  bool
 }
 
 // CreateLogger creates a logger with default settings.
@@ -82,6 +84,13 @@ func (l *Logger) SetJSONPretty(b bool) *Logger {
 	return l
 }
 
+// SetThresholdMode switches Logger.Print filtering from legacy severity-index
+// ordering to LogLevel threshold ordering.
+func (l *Logger) SetThresholdMode(b bool) *Logger {
+	l.threshold = b
+	return l
+}
+
 // AddWriters appends multiple writers to the logger.
 func (l *Logger) AddWriters(writers ...LoggerWriter) *Logger {
 	l.writers = append(l.writers, writers...)