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>
