(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

README.md

@@ -71,6 +71,10 @@ func main() {
 
 Important: with the current level ordering, `Level(sneklog.FATAL)` allows `INFO`, `WARN`, `ERROR`, and `FATAL`, but not `DEBUG`. Use `Level(sneklog.DEBUG)` to enable every level.
 
+If you want classic threshold filtering instead, call `SetThresholdMode(true)` and
+configure levels with `NewThresholdLogLevel(...)` or
+`NewThresholdLogLevelWithColors(...)`.
+
 ## Writers and ownership
 
 `Logger.Close()` only closes writers created by the logger itself:
@@ -142,6 +146,29 @@ access := sneklog.NewInfoLogLevelWithColors("access", sneklog.FgCyan, sneklog.Bg
 audit := sneklog.NewWarnLogLevel("audit")
 ```
 
+These helpers keep the legacy severity index for backward compatibility and also
+assign default threshold values:
+
+- `INFO`: `th=10`
+- `WARN`: `th=20`
+- `ERROR`: `th=30`
+- `FATAL`: `th=40`
+- `DEBUG`: `th=0`
+
+To define custom threshold-based levels explicitly:
+
+```go
+trace := sneklog.NewThresholdLogLevelWithColors(4, 5, "trace", sneklog.FgHiBlack, sneklog.BgNone)
+audit := sneklog.NewThresholdLogLevel(1, 25, "audit")
+
+logger := sneklog.NewLogger().
+	SetLevel(audit).
+	SetThresholdMode(true)
+
+logger.Print(trace, "verbose trace") // filtered out
+logger.Print(audit, "audit event")   // allowed
+```
+
 HTTP method-specific predefined levels are available out of the box:
 
 ```go
@@ -149,8 +176,11 @@ level := sneklog.LogLevelForMethod(http.MethodPost)
 logger.Print(level, "POST /users")
 ```
 
-When comparing levels, use `SameLevel` if only the severity matters and `Equal`
-if the full configuration, including colors and attributes, must match.
+When comparing levels:
+
+- use `SameLevel` if only the legacy severity index matters;
+- use `SameThreshold` if only threshold filtering matters;
+- use `Equal` if the full configuration, including threshold, colors, and attributes, must match.
 
 ## Message replacement