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>

pkg/router/middleware.go

@@ -0,0 +1,7 @@
+package router
+
+import "net/http"
+
+// Middleware is the standard func(http.Handler) http.Handler middleware shape,
+// compatible with any stdlib-style middleware.
+type Middleware = func(http.Handler) http.Handler