Add examples/ with runnable usage demos for all major features

Six examples covering the full API surface:
- basic-client: retry, timeout, logging, response size limit
- form-request: form-encoded POST for OAuth/webhooks
- load-balancing: weighted endpoints, circuit breaker, health checks
- server-basic: routing, groups, JSON helpers, health, custom 404
- server-protected: CORS, rate limiting, body limits, timeouts
- request-id-propagation: cross-service request ID forwarding

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

examples/server-protected/main.go

@@ -0,0 +1,61 @@
+// Production server with CORS, rate limiting, body size limits, and timeouts.
+package main
+
+import (
+	"log"
+	"log/slog"
+	"net/http"
+	"time"
+
+	"git.codelab.vc/pkg/httpx/server"
+)
+
+func main() {
+	logger := slog.Default()
+
+	r := server.NewRouter(
+		server.WithNotFoundHandler(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			server.WriteError(w, http.StatusNotFound, "not found")
+		})),
+	)
+
+	r.HandleFunc("GET /api/data", func(w http.ResponseWriter, _ *http.Request) {
+		server.WriteJSON(w, http.StatusOK, map[string]string{"status": "ok"})
+	})
+
+	r.HandleFunc("POST /api/upload", func(w http.ResponseWriter, r *http.Request) {
+		// Body is already limited by MaxBodySize middleware.
+		server.WriteJSON(w, http.StatusAccepted, map[string]string{"status": "received"})
+	})
+
+	r.Mount("/", server.HealthHandler())
+
+	srv := server.New(r,
+		append(
+			server.Defaults(logger),
+			server.WithMiddleware(
+				// CORS for browser-facing APIs.
+				server.CORS(
+					server.AllowOrigins("https://app.example.com", "https://admin.example.com"),
+					server.AllowMethods("GET", "POST", "PUT", "PATCH", "DELETE"),
+					server.AllowHeaders("Authorization", "Content-Type"),
+					server.ExposeHeaders("X-Request-Id"),
+					server.AllowCredentials(true),
+					server.MaxAge(3600),
+				),
+				// Rate limit: 100 req/s per IP, burst of 200.
+				server.RateLimit(
+					server.WithRate(100),
+					server.WithBurst(200),
+				),
+				// Limit request body to 1 MB.
+				server.MaxBodySize(1<<20),
+				// Per-request timeout of 30 seconds.
+				server.Timeout(30*time.Second),
+			),
+			server.WithAddr(":8080"),
+		)...,
+	)
+
+	log.Fatal(srv.ListenAndServe())
+}