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>

requestlog/requestlog.go

@@ -0,0 +1,61 @@
+// Package requestlog emits a structured "http.request" record per request
+// (method, path, status, duration, client) via splinter.
+package requestlog
+
+import (
+	"net/http"
+	"time"
+
+	"git.juancwu.dev/juancwu/splinter"
+)
+
+type statusRecorder struct {
+	http.ResponseWriter
+	status int
+	wrote  bool
+}
+
+func (s *statusRecorder) WriteHeader(code int) {
+	if !s.wrote {
+		s.status = code
+		s.wrote = true
+	}
+	s.ResponseWriter.WriteHeader(code)
+}
+
+func (s *statusRecorder) Write(b []byte) (int, error) {
+	if !s.wrote {
+		s.status = http.StatusOK
+		s.wrote = true
+	}
+	return s.ResponseWriter.Write(b)
+}
+
+// New returns a request-logging middleware. Pass nil to use splinter.Default()
+// resolved at request time; otherwise records flow through the supplied logger.
+func New(l *splinter.Logger) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			start := time.Now()
+			rec := &statusRecorder{ResponseWriter: w, status: http.StatusOK}
+			next.ServeHTTP(rec, r)
+			if l == nil {
+				splinter.Info("http.request",
+					"method", r.Method,
+					"path", r.URL.Path,
+					"status", rec.status,
+					"duration", time.Since(start),
+					"client", r.RemoteAddr,
+				)
+				return
+			}
+			l.Info("http.request",
+				"method", r.Method,
+				"path", r.URL.Path,
+				"status", rec.status,
+				"duration", time.Since(start),
+				"client", r.RemoteAddr,
+			)
+		})
+	}
+}