(new): prepare v2.1.0 release

- add custom LogLevel constructors and richer color configuration
- support ANSI, 256-color, RGB, and text attributes on LogLevel
- introduce clearer preferred APIs such as NewLogger and SetName
- preserve backward compatibility with deprecated wrappers for v2.0.1 APIs
- restore legacy ColorStringBuilder signatures as compatibility wrappers
- fix newline separator handling in Println-style output
- add tests for color precedence, deprecated aliases, and LogLevel attributes
- update GoDoc and bilingual README documentation

README.md

@@ -32,9 +32,9 @@ import (
 )
 
 func main() {
-	logger := sneklog.CreateLogger().
-		Prefix("API").
-		Level(sneklog.DEBUG).
+	logger := sneklog.NewLogger().
+		SetName("API").
+		SetLevel(sneklog.DEBUG).
 		AddReplacer("SOME_SECRET", "<redacted>")
 
 	text := logger.CreateTextStdoutWriter()
@@ -59,7 +59,7 @@ func main() {
 
 ## Defaults
 
-`CreateLogger()` starts with:
+`NewLogger()` starts with:
 
 - `Prefix("LOG")`
 - `Level(sneklog.FATAL)`
@@ -67,6 +67,8 @@ func main() {
 - text formatter: `sneklog.DefaultTextFormatter`
 - JSON formatter: `sneklog.DefaultJsonFormatter`
 
+`CreateLogger()` is still available for backward compatibility.
+
 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.
 
 ## Writers and ownership
@@ -117,6 +119,22 @@ JSON writers emit objects with this shape:
 
 When `JsonPretty(true)` is enabled, JSON is indented.
 
+## Custom levels and colors
+
+You can define your own levels and assign ANSI, 256-color, or RGB colors, plus text attributes such as `Bold` or `Italic`.
+
+```go
+httpDelete := sneklog.NewLogLevel(0, "delete")
+httpDelete.SetBackgroundRGB(128, 0, 0)
+httpDelete.AddAttribute(sneklog.Italic).AddAttribute(sneklog.Bold)
+
+httpCache := sneklog.NewLogLevel(0, "cache")
+httpCache.SetForeground256Color(214)
+```
+
+Because `LogLevel` setters mutate the level in place, call them on a variable, not on a temporary value returned by `NewLogLevel(...)`.
+Short forms such as `SetFgColor` and `SetBgColor` remain available for backward compatibility.
+
 ## Message replacement
 
 `AddReplacer(old, new)` replaces matching text in every message before the