Add production-ready HTTP server package with routing, health checks, and middleware

Introduces server/ sub-package as the server-side companion to the existing Client.
Includes Router (over http.ServeMux with groups and mounting), graceful shutdown with
signal handling, health endpoints (/healthz, /readyz), and built-in middlewares
(RequestID, Recovery, Logging). Zero external dependencies.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/middleware.go

@@ -0,0 +1,56 @@
+package server
+
+import "net/http"
+
+// Middleware wraps an http.Handler to add behavior.
+// This is the server-side counterpart of the client middleware type
+// func(http.RoundTripper) http.RoundTripper.
+type Middleware func(http.Handler) http.Handler
+
+// Chain composes middlewares so that Chain(A, B, C)(handler) == A(B(C(handler))).
+// Middlewares are applied from right to left: C wraps handler first, then B wraps
+// the result, then A wraps last. This means A is the outermost layer and sees
+// every request first.
+func Chain(mws ...Middleware) Middleware {
+	return func(h http.Handler) http.Handler {
+		for i := len(mws) - 1; i >= 0; i-- {
+			h = mws[i](h)
+		}
+		return h
+	}
+}
+
+// statusWriter wraps http.ResponseWriter to capture the response status code.
+// It implements Unwrap() so that http.ResponseController can access the
+// underlying ResponseWriter's optional interfaces (Flusher, Hijacker, etc.).
+type statusWriter struct {
+	http.ResponseWriter
+	status      int
+	written     bool
+}
+
+// WriteHeader captures the status code and delegates to the underlying writer.
+func (w *statusWriter) WriteHeader(code int) {
+	if !w.written {
+		w.status = code
+		w.written = true
+	}
+	w.ResponseWriter.WriteHeader(code)
+}
+
+// Write delegates to the underlying writer, defaulting status to 200 if
+// WriteHeader was not called explicitly.
+func (w *statusWriter) Write(b []byte) (int, error) {
+	if !w.written {
+		w.status = http.StatusOK
+		w.written = true
+	}
+	return w.ResponseWriter.Write(b)
+}
+
+// Unwrap returns the underlying ResponseWriter. This is required for
+// http.ResponseController to detect optional interfaces like http.Flusher
+// and http.Hijacker on the original writer.
+func (w *statusWriter) Unwrap() http.ResponseWriter {
+	return w.ResponseWriter
+}