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/form-request/main.go

@@ -0,0 +1,41 @@
+// Demonstrates form-encoded requests for OAuth token endpoints and similar APIs.
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"net/http"
+	"net/url"
+
+	"git.codelab.vc/pkg/httpx"
+)
+
+func main() {
+	client := httpx.New(
+		httpx.WithBaseURL("https://httpbin.org"),
+	)
+	defer client.Close()
+
+	ctx := context.Background()
+
+	// Form-encoded POST, common for OAuth token endpoints.
+	req, err := httpx.NewFormRequest(ctx, http.MethodPost, "/post", url.Values{
+		"grant_type":    {"client_credentials"},
+		"client_id":     {"my-app"},
+		"client_secret": {"secret"},
+		"scope":         {"read write"},
+	})
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	resp, err := client.Do(ctx, req)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer resp.Close()
+
+	body, _ := resp.String()
+	fmt.Printf("Status: %d\nBody: %s\n", resp.StatusCode, body)
+}