(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

RELEASE_NOTES.md

@@ -1,6 +1,8 @@
 # Unreleased (planned v2.2.0)
 
-This release expands the public LogLevel API, adds HTTP method helpers, and improves package documentation and test coverage.
+This release expands the public LogLevel API, adds optional threshold-based
+filtering alongside the legacy severity ordering, and improves documentation
+and test coverage.
 
 ### Added
 
@@ -15,6 +17,9 @@ This release expands the public LogLevel API, adds HTTP method helpers, and impr
   - `NewFatalLogLevelWithColors`
   - `NewDebugLogLevel`
   - `NewDebugLogLevelWithColors`
+- Added threshold-aware log level constructors:
+  - `NewThresholdLogLevel`
+  - `NewThresholdLogLevelWithColors`
 - Added HTTP method-specific predefined log levels:
   - `HTTPGetLevel`
   - `HTTPHeadLevel`
@@ -28,24 +33,30 @@ This release expands the public LogLevel API, adds HTTP method helpers, and impr
   - `HTTPUnknownLevel`
 - Added `LogLevelForMethod(method string) LogLevel` to map HTTP methods to predefined log levels.
 - Added `Logger.Printf(level, format, args...)` for formatted logging with an explicit log level.
-- Added `LogLevel.SameLevel` and `LogLevel.Equal` for severity-only and full level comparisons.
+- Added `Logger.SetThresholdMode(bool)` to switch filtering from legacy severity-index ordering to threshold-based ordering.
+- Added `LogLevel.SameLevel`, `LogLevel.SameThreshold`, and `LogLevel.Equal` for severity-only, threshold-only, and full level comparisons.
 
 ### Changed
 
 - Refactored log level definitions into a dedicated `levels.go` file.
 - Unified `Logger.Print` and `Logger.Println` through a shared internal implementation without changing their behavior.
-- Expanded test coverage for `Logger.Printf`, HTTP level mapping, and new level constructors.
+- Common helper constructors such as `NewInfoLogLevel` now also assign threshold values while preserving legacy severity indexes.
+- `LogLevel.Equal` now includes threshold configuration in full equality checks.
+- Expanded test coverage for `Logger.Printf`, HTTP level mapping, constructor behavior, threshold filtering, and backward compatibility.
 
 ### Documentation
 
 - Improved GoDoc coverage for exported types, constructors, formatters, colors, HTTP helpers, and time layouts.
-- Documented level comparison helpers and explicit-level formatted logging.
+- Documented threshold mode, threshold-aware constructors, level comparison helpers, and explicit-level formatted logging.
 - Expanded inline documentation for formatter tokens and default formatter behavior.
+- Updated English and Russian README files with threshold-mode examples and comparison guidance.
 
 ### Compatibility
 
 - No breaking API changes.
-- Existing `Print`, `Println`, predefined levels, and color APIs remain compatible.
+- Existing `Print`, `Println`, predefined levels, `Level/SetLevel`, and color APIs remain compatible.
+- Legacy severity ordering remains the default; threshold filtering is enabled only when `SetThresholdMode(true)` is called.
+- `SameLevel` remains severity-only for backward compatibility, even when levels carry threshold metadata.
 
 # v2.1.0