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>

README.md

@@ -195,6 +195,19 @@ client := httpx.New(
 )
 ```
 
+## Examples
+
+See the [`examples/`](examples/) directory for runnable programs:
+
+| Example | Description |
+|---------|-------------|
+| [`basic-client`](examples/basic-client/) | HTTP client with retry, timeout, logging, and response size limit |
+| [`form-request`](examples/form-request/) | Form-encoded POST requests (OAuth, webhooks) |
+| [`load-balancing`](examples/load-balancing/) | Multi-endpoint client with weighted balancing, circuit breaker, and health checks |
+| [`server-basic`](examples/server-basic/) | Server with routing, groups, JSON helpers, health checks, and custom 404 |
+| [`server-protected`](examples/server-protected/) | Production server with CORS, rate limiting, body limits, and timeouts |
+| [`request-id-propagation`](examples/request-id-propagation/) | Request ID forwarding between server and client for distributed tracing |
+
 ## Requirements
 
 Go 1.24+, stdlib only.