(refactor): prepare breaking v2 API

- rename package to sneklog and move module to /v2
- replace text output flags with writer formatters
- remove external color dependencies
- update migration notes and coverage

README.md

@@ -1,4 +1,4 @@
-# slog
+# SNekLog (ScuroNeko Logger)
 
 Small structured logger for Go with text and JSON output, multiple writers, and optional traceback metadata.
 
@@ -17,7 +17,7 @@ Russian version: [README_ru.md](README_ru.md)
 ## Installation
 
 ```bash
-go get git.scuroneko.dev/scuroneko/slog
+go get git.scuroneko.dev/scuroneko/slog/v2
 ```
 
 ## Quick start
@@ -28,15 +28,13 @@ package main
 import (
 	"log"
 
-	"git.scuroneko.dev/scuroneko/slog"
+	"git.scuroneko.dev/scuroneko/slog/v2"
 )
 
 func main() {
-	logger := slog.CreateLogger().
+	logger := sneklog.CreateLogger().
 		Prefix("API").
-		Level(slog.DEBUG).
-		PrintTraceback(true).
-		JsonPretty(true).
+		Level(sneklog.DEBUG).
 		AddReplacer("SOME_SECRET", "<redacted>")
 
 	text := logger.CreateTextStdoutWriter()
@@ -64,12 +62,12 @@ func main() {
 `CreateLogger()` starts with:
 
 - `Prefix("LOG")`
-- `Level(slog.FATAL)`
-- `PrintTime(true)`
-- `PrintTraceback(false)`
+- `Level(sneklog.FATAL)`
 - `JsonPretty(false)`
+- text formatter: `sneklog.DefaultTextFormatter`
+- JSON formatter: `sneklog.DefaultJsonFormatter`
 
-Important: with the current level ordering, `Level(slog.FATAL)` allows `INFO`, `WARN`, `ERROR`, and `FATAL`, but not `DEBUG`. Use `Level(slog.DEBUG)` to enable every level.
+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
 
@@ -92,10 +90,10 @@ This makes it safe to plug in `bytes.Buffer`, network writers, and other externa
 Text writers render records like:
 
 ```text
-[API] [INFO] [main.go:main:27] [17.03.26 14:05:09] service started
+2026-03-17T14:05:09+03:00 info API: service started
 ```
 
-The traceback field is included only when `PrintTraceback(true)` is enabled.
+Use `SetFormatter` on a writer to customize timestamps, traceback fields, colors, and message layout.
 
 JSON writers emit objects with this shape:
 
@@ -126,8 +124,8 @@ record reaches any writer. Replacement rules are applied in the order they are
 added.
 
 ```go
-logger := slog.CreateLogger().
-	Level(slog.DEBUG).
+logger := sneklog.CreateLogger().
+	Level(sneklog.DEBUG).
 	AddReplacer("SOME_SECRET", "<redacted>").
 	AddReplacer("user@example.com", "<email>")