(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

levels.go

@@ -11,8 +11,9 @@ var (
 
 // LogLevel describes a logging severity.
 type LogLevel struct {
-	n uint8
-	t string
+	n  uint8
+	th uint8
+	t  string
 
 	fg    FgColor
 	fg256 FgColor256
@@ -26,53 +27,68 @@ type LogLevel struct {
 }
 
 // NewLogLevel creates a log level without predefined colors.
+// Deprecated: use NewThresholdLogLevel. This function will be removed in v3.
 func NewLogLevel(index uint8, name string) LogLevel {
 	return NewLogLevelWithColors(index, name, 0, 0)
 }
 
 // NewLogLevelWithColors creates a log level with ANSI foreground and background colors.
+// Deprecated: use NewThresholdLogLevelWithColors. This function will be removed in v3.
 func NewLogLevelWithColors(index uint8, name string, fg FgColor, bg BgColor) LogLevel {
 	return LogLevel{n: index, t: name, fg: fg, bg: bg, attrs: []Attribute{}}
 }
 
+// NewThresholdLogLevel creates a log level with both a legacy severity index and
+// a threshold value used when Logger.SetThresholdMode(true) is enabled.
+func NewThresholdLogLevel(index, th uint8, name string) LogLevel {
+	return NewThresholdLogLevelWithColors(index, th, name, FgNone, BgNone)
+}
+
+// NewThresholdLogLevelWithColors creates a log level with both a legacy
+// severity index and a threshold value, plus ANSI foreground and background
+// colors.
+func NewThresholdLogLevelWithColors(index, th uint8, name string, fg FgColor, bg BgColor) LogLevel {
+	return LogLevel{n: index, th: th, t: name, fg: fg, bg: bg, attrs: []Attribute{}}
+}
+
 // NewInfoLogLevel creates an info-level log level without predefined colors.
-func NewInfoLogLevel(name string) LogLevel { return NewLogLevel(0, name) }
+func NewInfoLogLevel(name string) LogLevel { return NewThresholdLogLevel(0, 10, name) }
 
 // NewInfoLogLevelWithColors creates an info-level log level with ANSI colors.
 func NewInfoLogLevelWithColors(name string, fg FgColor, bg BgColor) LogLevel {
-	return NewLogLevelWithColors(0, name, fg, bg)
+	return NewThresholdLogLevelWithColors(0, 10, name, fg, bg)
 }
 
 // NewWarnLogLevel creates a warn-level log level without predefined colors.
-func NewWarnLogLevel(name string) LogLevel { return NewLogLevel(1, name) }
+func NewWarnLogLevel(name string) LogLevel { return NewThresholdLogLevel(1, 20, name) }
 
 // NewWarnLogLevelWithColors creates a warn-level log level with ANSI colors.
 func NewWarnLogLevelWithColors(name string, fg FgColor, bg BgColor) LogLevel {
-	return NewLogLevelWithColors(1, name, fg, bg)
+	return NewThresholdLogLevelWithColors(1, 20, name, fg, bg)
 }
 
 // NewErrorLogLevel creates an error-level log level without predefined colors.
-func NewErrorLogLevel(name string) LogLevel { return NewLogLevel(2, name) }
+func NewErrorLogLevel(name string) LogLevel { return NewThresholdLogLevel(2, 30, name) }
 
 // NewErrorLogLevelWithColors creates an error-level log level with ANSI colors.
 func NewErrorLogLevelWithColors(name string, fg FgColor, bg BgColor) LogLevel {
-	return NewLogLevelWithColors(2, name, fg, bg)
+	return NewThresholdLogLevelWithColors(2, 30, name, fg, bg)
 }
 
 // NewFatalLogLevel creates a fatal-level log level without predefined colors.
-func NewFatalLogLevel(name string) LogLevel { return NewLogLevel(3, name) }
+func NewFatalLogLevel(name string) LogLevel { return NewThresholdLogLevel(3, 40, name) }
 
 // NewFatalLogLevelWithColors creates a fatal-level log level with ANSI colors.
 func NewFatalLogLevelWithColors(name string, fg FgColor, bg BgColor) LogLevel {
-	return NewLogLevelWithColors(3, name, fg, bg)
+	return NewThresholdLogLevelWithColors(3, 40, name, fg, bg)
 }
 
 // NewDebugLogLevel creates a debug-level log level without predefined colors.
-func NewDebugLogLevel(name string) LogLevel { return NewLogLevel(4, name) }
+func NewDebugLogLevel(name string) LogLevel { return NewThresholdLogLevel(4, 0, name) }
 
 // NewDebugLogLevelWithColors creates a debug-level log level with ANSI colors.
 func NewDebugLogLevelWithColors(name string, fg FgColor, bg BgColor) LogLevel {
-	return NewLogLevelWithColors(4, name, fg, bg)
+	return NewThresholdLogLevelWithColors(4, 0, name, fg, bg)
 }
 
 // GetName returns the lowercase textual representation of the level.
@@ -198,13 +214,21 @@ func (l *LogLevel) GetAttributes() []Attribute {
 }
 
 // SameLevel reports whether two log levels have the same severity index.
+// Deprecated: use SameThreshold for threshold-only comparisons or Equal for the
+// full level configuration. SameLevel remains severity-only for backward
+// compatibility and ignores threshold values.
 func (l *LogLevel) SameLevel(other LogLevel) bool {
 	return l.n == other.n
 }
 
+// SameThreshold reports whether two log levels have the same threshold value.
+func (l *LogLevel) SameThreshold(other LogLevel) bool {
+	return l.th == other.th
+}
+
 // Equal reports whether two log levels have identical configuration.
 func (l *LogLevel) Equal(other LogLevel) bool {
-	if l.t != other.t || l.n != other.n {
+	if l.t != other.t || l.n != other.n || l.th != other.th {
 		return false
 	}
 	if l.fg != other.fg || l.bg != other.bg {