(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

RELEASE_NOTES.md

@@ -1,3 +1,30 @@
+# v2.1.0
+
+This release expands sneklog color customization while preserving compatibility with the v2.0.1 API.
+
+## Added
+- custom LogLevel constructors with configurable colors
+- 256-color and RGB support for LogLevel
+- ANSI text attributes on LogLevel such as Bold and Italic
+- typed color helpers for foreground/background RGB and 256-color values
+- NewLogger, SetName, SetLevel, and SetJSONPretty as clearer preferred APIs
+
+## Improved
+- text and JSON writer handling of trailing newline semantics
+- formatter colorization logic for foreground, background, and attributes
+- GoDoc coverage across the public API
+- English and Russian README examples and API guidance
+
+## Compatibility
+- legacy APIs from v2.0.1 remain available as deprecated wrappers
+- legacy ColorStringBuilder method signatures were preserved for backward compatibility
+- deprecated methods now explicitly document their replacements and planned removal in v3
+
+## Fixed
+- extra separator/space issues in Println-style output
+- attribute slice handling and defensive copying for LogLevel
+- color mode precedence when switching between ANSI, 256-color, and RGB modes
+
 # v2.0.0
 
 ## Highlights