add realip, requestlog, recoverer middlewares

Initial implementation of lightmux-contrib, a sibling module to
lightmux that hosts opinionated middlewares with one sub-package per
middleware:

- realip: resolves the originating client IP from CF-Connecting-IP,
  True-Client-IP, X-Real-IP, or X-Forwarded-For. Optional peer-CIDR
  allowlist via netip.Prefix.
- requestlog: emits a structured http.request record (method, path,
  status, duration, client) per request via splinter.
- recoverer: catches panics, wraps with errx under op "recoverer",
  logs with stack, and writes a 500 response.

Each package exposes a single New(...) constructor returning
func(http.Handler) http.Handler. The contrib module intentionally
does not import lightmux — middlewares interoperate via the standard
stdlib middleware shape.

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

recoverer/recoverer.go

@@ -0,0 +1,36 @@
+// Package recoverer catches panics inside HTTP handlers, logs them with stack
+// trace, and writes a 500 response.
+package recoverer
+
+import (
+	"log"
+	"net/http"
+	"runtime/debug"
+
+	"git.juancwu.dev/juancwu/errx"
+)
+
+const op = "recoverer"
+
+// New returns a panic-recovery middleware.
+func New() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			defer func() {
+				rec := recover()
+				if rec == nil {
+					return
+				}
+				var err error
+				if e, ok := rec.(error); ok {
+					err = errx.Wrap(op, e)
+				} else {
+					err = errx.Newf(op, "panic: %v", rec)
+				}
+				log.Printf("%v\n%s", err, debug.Stack())
+				http.Error(w, "Internal Server Error", http.StatusInternalServerError)
+			}()
+			next.ServeHTTP(w, r)
+		})
+	}
+}