extract middlewares to lightmux-contrib

Removes pkg/middleware. The Logger, Recoverer, and RealIP middlewares
now live in the sibling lightmux-contrib module as the realip,
requestlog, and recoverer packages, each exposing a single New(...)
constructor.

The Middleware type alias moves to pkg/router. The splinter dependency
is dropped from go.mod; only errx remains.

BREAKING CHANGE: consumers must replace pkg/middleware imports with
the corresponding lightmux-contrib sub-packages. See README for the
new usage.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lightmux.go

@@ -2,14 +2,11 @@
 // adding method-named convenience methods, groups, and per-route middleware.
 package lightmux
 
-import (
-	"git.juancwu.dev/juancwu/lightmux/pkg/middleware"
-	"git.juancwu.dev/juancwu/lightmux/pkg/router"
-)
+import "git.juancwu.dev/juancwu/lightmux/pkg/router"
 
 type (
 	Mux        = router.Mux
-	Middleware = middleware.Middleware
+	Middleware = router.Middleware
 )
 
 func New() *Mux { return router.New() }