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/load-balancing/main.go

@@ -0,0 +1,54 @@
+// Demonstrates load balancing across multiple backend endpoints with
+// circuit breaking and health-checked failover.
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"log/slog"
+	"time"
+
+	"git.codelab.vc/pkg/httpx"
+	"git.codelab.vc/pkg/httpx/balancer"
+	"git.codelab.vc/pkg/httpx/circuitbreaker"
+	"git.codelab.vc/pkg/httpx/retry"
+)
+
+func main() {
+	client := httpx.New(
+		httpx.WithEndpoints(
+			balancer.Endpoint{URL: "http://localhost:8081", Weight: 3},
+			balancer.Endpoint{URL: "http://localhost:8082", Weight: 1},
+		),
+		httpx.WithBalancer(
+			balancer.WithStrategy(balancer.WeightedRandom()),
+			balancer.WithHealthCheck(
+				balancer.WithHealthInterval(5*time.Second),
+				balancer.WithHealthPath("/healthz"),
+			),
+		),
+		httpx.WithCircuitBreaker(
+			circuitbreaker.WithFailureThreshold(5),
+			circuitbreaker.WithOpenDuration(30*time.Second),
+		),
+		httpx.WithRetry(
+			retry.WithMaxAttempts(3),
+		),
+		httpx.WithLogger(slog.Default()),
+	)
+	defer client.Close()
+
+	ctx := context.Background()
+
+	for i := range 5 {
+		resp, err := client.Get(ctx, fmt.Sprintf("/api/item/%d", i))
+		if err != nil {
+			log.Printf("request %d failed: %v", i, err)
+			continue
+		}
+
+		body, _ := resp.String()
+		fmt.Printf("request %d → %d: %s\n", i, resp.StatusCode, body)
+	}
+}