refactor(logger): replace logrus with stdlib log/slog

Drop the external github.com/sirupsen/logrus dependency in favor of Go's
standard log/slog package (available since Go 1.21). The core/logger.Logger
interface is preserved unchanged — this is a pure implementation swap.

Key changes:
- slogLogger replaces logrusLogger, backed by slog.Logger + slog.LevelVar
- Custom levelTrace (-8) and levelFatal (+12) constants extend slog's
  built-in 4 levels to cover the 6-level logrus-compatible range
- replaceAttr normalises output: lowercase level names and RFC 3339
  timestamps with milliseconds (matches previous logrus JSON format)
- Shared *slog.LevelVar ensures WithFields clones inherit parent level
- Level guard (early return) in log/logf avoids wasted fmt.Sprint/Sprintf
  when the message level is below the configured threshold
- Documented the stack depth assumption for the caller skip=3 frame

Test coverage (14 tests, all passing with -race):
- convertLevel/revertLevel round-trip for all 6 levels
- levelString for all levels including custom trace/fatal
- replaceAttr timestamp format, level name, groups passthrough
- caller format, level guard, WithFields, IsLevelEnabled, GetLevel

go.mod

@@ -30,7 +30,6 @@ require (
 	github.com/quic-go/webtransport-go v0.10.0
 	github.com/rs/xid v1.3.0
 	github.com/shadowsocks/go-shadowsocks2 v0.1.5
-	github.com/sirupsen/logrus v1.9.3
 	github.com/songgao/water v0.0.0-20200317203138-2b4b6d7c09d8
 	github.com/spf13/viper v1.19.0
 	github.com/stretchr/testify v1.11.1
@@ -57,6 +56,8 @@ require (
 	gvisor.dev/gvisor v0.0.0-20250523182742-eede7a881b20
 )
 
+require github.com/sirupsen/logrus v1.9.3 // indirect
+
 require (
 	github.com/alessio/shellescape v1.4.1 // indirect
 	github.com/beorn7/perks v1.0.1 // indirect